Detect C standard and try using the most recent one (take 2)

Fixes the default C version used with gcc < 5

Change-Id: I948dece961caed8e6b181e1c6e6b9dc43c46583e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

11 files changed, 103 insertions, 7 deletions

diff --git a/configure.json b/configure.json
index cc78938173..323b25e1d6 100644
--- a/configure.json
+++ b/configure.json
@@ -260,6 +260,36 @@
             "type": "compile",
             "test": "stl"
         },
+        "c99": {
+            "label": "C99 support",
+            "type": "compile",
+            "test": {
+                "head": [
+                    "#if __STDC_VERSION__ >= 199901L",
+                    "// Compiler claims to support C99, trust it",
+                    "#else",
+                    "#  error __STDC_VERSION__ must be >= 199901L",
+                    "#endif"
+                ],
+                "lang": "c",
+                "qmake": "CONFIG += c99"
+            }
+        },
+        "c11": {
+            "label": "C11 support",
+            "type": "compile",
+            "test": {
+                "head": [
+                    "#if __STDC_VERSION__ >= 201112L",
+                    "// Compiler claims to support C11, trust it",
+                    "#else",
+                    "#  error __STDC_VERSION__ must be >= 201112L",
+                    "#endif"
+                ],
+                "lang": "c",
+                "qmake": "CONFIG += c11"
+            }
+        },
         "c++14": {
             "label": "C++14 support",
             "type": "compile",
@@ -787,6 +817,19 @@
             "condition": "features.c++14 && tests.c++1z",
             "output": [ "publicFeature", "publicQtConfig" ]
         },
+        "c89": {
+            "label": "C89"
+        },
+        "c99": {
+            "label": "C99",
+            "condition": "tests.c99",
+            "output": [ "publicFeature" ]
+        },
+        "c11": {
+            "label": "C11",
+            "condition": "features.c99 && tests.c11",
+            "output": [ "publicFeature" ]
+        },
         "precompile_header": {
             "label": "Using precompiled headers",
             "condition": "config.msvc || tests.precompile_header",
@@ -1248,6 +1291,11 @@ Configure with '-qreal float' to create a build that is binary-compatible with 5
                 },
                 "shared",
                 {
+                    "message": "Using C standard",
+                    "type": "firstAvailableFeature",
+                    "args": "c11 c99 c89"
+                },
+                {
                     "message": "Using C++ standard",
                     "type": "firstAvailableFeature",
                     "args": "c++1z c++14 c++11"
diff --git a/mkspecs/common/android-base-tail.conf b/mkspecs/common/android-base-tail.conf
index 7c3ae9566a..e239fa01c5 100644
--- a/mkspecs/common/android-base-tail.conf
+++ b/mkspecs/common/android-base-tail.conf
@@ -30,6 +30,8 @@ QMAKE_CFLAGS_THREAD     = -D_REENTRANT
 QMAKE_CFLAGS_HIDESYMS   = -fvisibility=hidden
 QMAKE_CFLAGS_NEON       = -mfpu=neon
 
+QMAKE_CFLAGS_GNUC99     = -std=gnu99
+QMAKE_CFLAGS_GNUC11     = -std=gnu11
 QMAKE_CXXFLAGS_CXX11    = -std=c++11
 QMAKE_CXXFLAGS_CXX14    = -std=c++14
 QMAKE_CXXFLAGS_CXX1Z    = -std=c++1z
diff --git a/mkspecs/common/clang.conf b/mkspecs/common/clang.conf
index e003b947aa..6122a37213 100644
--- a/mkspecs/common/clang.conf
+++ b/mkspecs/common/clang.conf
@@ -21,6 +21,8 @@ QMAKE_CFLAGS_PRECOMPILE          = -x c-header -c ${QMAKE_PCH_INPUT} -o ${QMAKE_
 QMAKE_CFLAGS_USE_PRECOMPILE      = -Xclang -include-pch -Xclang ${QMAKE_PCH_OUTPUT}
 QMAKE_CFLAGS_LTCG                = -flto
 QMAKE_CFLAGS_DISABLE_LTCG        = -fno-lto
+QMAKE_CFLAGS_GNUC99              = -std=gnu99
+QMAKE_CFLAGS_GNUC11              = -std=gnu11
 
 QMAKE_CXXFLAGS_PRECOMPILE        = -x c++-header -c ${QMAKE_PCH_INPUT} -o ${QMAKE_PCH_OUTPUT}
 QMAKE_CXXFLAGS_USE_PRECOMPILE    = $$QMAKE_CFLAGS_USE_PRECOMPILE
diff --git a/mkspecs/common/g++-base.conf b/mkspecs/common/g++-base.conf
index c42c46b0ec..fa0f0c391d 100644
--- a/mkspecs/common/g++-base.conf
+++ b/mkspecs/common/g++-base.conf
@@ -27,6 +27,8 @@ QMAKE_CFLAGS_USE_PRECOMPILE   = -include ${QMAKE_PCH_OUTPUT_BASE}
 QMAKE_CXXFLAGS_PRECOMPILE     = -x c++-header -c ${QMAKE_PCH_INPUT} -o ${QMAKE_PCH_OUTPUT}
 QMAKE_CXXFLAGS_USE_PRECOMPILE = $$QMAKE_CFLAGS_USE_PRECOMPILE
 
+QMAKE_CFLAGS_GNUC99     = -std=gnu99
+QMAKE_CFLAGS_GNUC11     = -std=gnu11
 QMAKE_CXXFLAGS_CXX11    = -std=c++11
 QMAKE_CXXFLAGS_CXX14    = -std=c++1y
 QMAKE_CXXFLAGS_CXX1Z    = -std=c++1z
diff --git a/mkspecs/common/gcc-base.conf b/mkspecs/common/gcc-base.conf
index 234f71d495..0407ba9724 100644
--- a/mkspecs/common/gcc-base.conf
+++ b/mkspecs/common/gcc-base.conf
@@ -81,6 +81,9 @@ QMAKE_LFLAGS_RELEASE_WITH_DEBUGINFO +=
 QMAKE_LFLAGS_EXCEPTIONS_OFF +=
 QMAKE_LFLAGS_LTCG       = $$QMAKE_CFLAGS_LTCG -fuse-linker-plugin
 
+QMAKE_CFLAGS_C99       = -std=c99
+QMAKE_CFLAGS_C11       = -std=c11
+
 QMAKE_CFLAGS_SSE2      += -msse2
 QMAKE_CFLAGS_SSE3      += -msse3
 QMAKE_CFLAGS_SSSE3     += -mssse3
diff --git a/mkspecs/features/default_post.prf b/mkspecs/features/default_post.prf
index 5367b17973..ad4a5f6365 100644
--- a/mkspecs/features/default_post.prf
+++ b/mkspecs/features/default_post.prf
@@ -143,6 +143,19 @@ c++11|c++14|c++1z {
     unset(cxxstd)
 }
 
+c99|c11 {
+    c11:       cstd = C11
+    else:      cstd = C99
+
+    !isEmpty(QMAKE_CFLAGS_GNU$$cstd) {
+        !strict_c|isEmpty(QMAKE_CFLAGS_$$cstd): cstd = GNU$$cstd
+    }
+
+    QMAKE_CFLAGS += $$eval(QMAKE_CFLAGS_$$cstd)
+
+    unset(cstd)
+}
+
 utf8_source {
     QMAKE_CFLAGS += $$QMAKE_CFLAGS_UTF8_SOURCE
     QMAKE_CXXFLAGS += $$QMAKE_CFLAGS_UTF8_SOURCE
diff --git a/mkspecs/features/qt_common.prf b/mkspecs/features/qt_common.prf
index 93f9a2429d..638fbf38be 100644
--- a/mkspecs/features/qt_common.prf
+++ b/mkspecs/features/qt_common.prf
@@ -17,6 +17,8 @@ DEFINES *= QT_NO_NARROWING_CONVERSIONS_IN_CONNECT
 qtConfig(c++11): CONFIG += c++11 strict_c++
 qtConfig(c++14): CONFIG += c++14
 qtConfig(c++1z): CONFIG += c++1z
+qtConfig(c99): CONFIG += c99
+qtConfig(c11): CONFIG += c11
 qtConfig(stack-protector-strong): CONFIG += stack_protector_strong
 contains(TEMPLATE, .*lib) {
     # module and plugins
diff --git a/mkspecs/features/qt_configure.prf b/mkspecs/features/qt_configure.prf
index 6e29eaa2b7..70c763a2c3 100644
--- a/mkspecs/features/qt_configure.prf
+++ b/mkspecs/features/qt_configure.prf
@@ -870,6 +870,15 @@ defineTest(qtConfTestPrepare_compile) {
 defineTest(qtConfPrepareCompileTestSource) {
     test_dir = $$2
 
+    test_lang = $$eval($${1}.lang)
+    isEmpty(test_lang): test_lang = "c++"
+
+    equals(test_lang, "c++"): suffix = "cpp"
+    else: equals(test_lang, "c"): suffix = "c"
+    else: equals(test_lang, "objc"): suffix = "m"
+    else: equals(test_lang, "objc++"): suffix = "mm"
+    else: error("Unknown language '$$test_lang' in compile test $$1")
+
     # Create source code
     contents = "/* Generated by configure */"
     # Custom code before includes
@@ -893,10 +902,10 @@ defineTest(qtConfPrepareCompileTestSource) {
         "    /* END TEST */" \
         "    return 0;" \
         "}"
-    write_file($$test_dir/main.cpp, contents)|error()
+    write_file($$test_dir/main.$$suffix, contents)|error()
 
     # Create stub .pro file
-    contents = "SOURCES = main.cpp"
+    contents = "SOURCES = main.$$suffix"
     # Custom project code
     for (ent, $$qtConfScalarOrList($${1}.qmake)): \
         contents += $$ent
diff --git a/qmake/doc/src/qmake-manual.qdoc b/qmake/doc/src/qmake-manual.qdoc
index 35f24e1793..9968528b56 100644
--- a/qmake/doc/src/qmake-manual.qdoc
+++ b/qmake/doc/src/qmake-manual.qdoc
@@ -956,12 +956,25 @@
             default is used.
     \row    \li thread \li Thread support is enabled. This is enabled when CONFIG
             includes \c qt, which is the default.
+    \row    \li c99 \li C99 support is enabled. This option has no effect if
+            the compiler does not support C99, or can't select the C standard.
+            By default, the compiler default is used.
+    \row    \li c11 \li C11 support is enabled. This option has no effect if
+            the compiler does not support C11, or can't select the C standard.
+            By default, the compiler default is used.
+    \row    \li strict_c Disables support for C compiler extensions.
+            By default, they are enabled.
     \row    \li c++11 \li C++11 support is enabled. This option has no effect if
-            the compiler does not support C++11.
-            By default, support is disabled.
+            the compiler does not support C++11, or can't select the C++ standard.
+            By default, support is enabled.
     \row    \li c++14 \li C++14 support is enabled. This option has no effect if
-            the compiler does not support C++14.
+            the compiler does not support C++14, or can't select the C++ standard.
+            By default, the compiler default is used.
+    \row    \li c++1z \li C++17 support is enabled. This option has no effect if
+            the compiler does not support C++17, or can't select the C++ standard.
             By default, support is disabled.
+    \row    \li strict_c++ Disables support for C++ compiler extensions.
+            By default, they are enabled.
     \row    \li depend_includepath \li Appending the value of INCLUDEPATH to
             DEPENDPATH is enabled. Set by default.
     \endtable
diff --git a/src/3rdparty/xkbcommon.pri b/src/3rdparty/xkbcommon.pri
index 1d953d8372..8259a8b8bd 100644
--- a/src/3rdparty/xkbcommon.pri
+++ b/src/3rdparty/xkbcommon.pri
@@ -1,4 +1,6 @@
-QMAKE_CFLAGS += -std=gnu99 -w
+# Requires GNU C extensions
+CONFIG -= strict_c
+
 INCLUDEPATH += $$PWD/xkbcommon \
                $$PWD/xkbcommon/xkbcommon \
                $$PWD/xkbcommon/src \
diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index f3f3139d1a..e5e47082d8 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -898,7 +898,7 @@
        /* C11 features supported in GCC 4.7: */
 #      define Q_COMPILER_STATIC_ASSERT
 #    endif
-#    if Q_CC_GNU >= 409
+#    if Q_CC_GNU >= 409 && defined(__has_include)
        /* C11 features supported in GCC 4.9: */
 #      if __has_include(<threads.h>)
 #        define Q_COMPILER_THREAD_LOCAL