CMake: Don't use libraries in /usr/local by default on macOS

qmake builds of Qt don't use libraries in /usr/local because the
path is not considered a system path. Only the SDK path should be
used as a source of system libraries.

We should do the same for the CMake builds, which involves a couple of
things.

Tell CMake not to consider /usr/local (and a bunch of other
paths) as system prefix paths.

Disable pkg-config usage which by default is not used in qmake
Windows and macOS builds.

If a user wishes to use libraries located in /usr/local on macOS, they
can explicitly enable the behavior via -DFEATURE_pkg_config=ON.

In addition to enabling pkg-config, that will also disable the system
prefix modification described above.

Implementation notes

To disable pkg-config usage, we set an empty path for
PKG_CONFIG_EXECUTABLE, because there is no other good way. The
downside to this is that a lot of warning messages will be printed
that the pkg-config package can not be found.

The pkg-config feature needs to be computed in QtBuildInternals before
qtbase/src/configure.cmake, because it's too late to do it in that
file where a few qt_find_package calls already exist.
The feature value is also saved to QtBuildInternalsExtra, to make sure
that pkg-config is disabled whenever building another repo.

System prefix adjustment is done by removing paths from
CMAKE_SYSTEM_PREFIX_PATH.
Ideally we would remove also /usr as a path, to match what qmake does,
but that breaks find_program() calls for perl, python, etc.

We have to make sure that qt_find_package does not look in
PATH when looking for packages, which is the default behavior, because
PATH on macOS most likely contains /usr/local.

One last curiosity for future generations is that CMake 3.18+ has
merged a change to prioritise SDK locations over regular /usr/lib.

Fixes: QTBUG-85261
Change-Id: I28fe5bae7997507a83b37b4eb1e0188e64062c57
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 14 insertions, 0 deletions

diff --git a/cmake/QtBuild.cmake b/cmake/QtBuild.cmake
index 4ddb61aec3..9656fa293a 100644
--- a/cmake/QtBuild.cmake
+++ b/cmake/QtBuild.cmake
@@ -5246,6 +5246,12 @@ macro(qt_find_package)
         list(APPEND arg_UNPARSED_ARGUMENTS "COMPONENTS;${arg_COMPONENTS}")
     endif()
 
+    # Don't look for packages in PATH if requested to.
+    if(QT_NO_USE_FIND_PACKAGE_SYSTEM_ENVIRONMENT_PATH)
+        set(_qt_find_package_use_system_env_backup "${CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH}")
+        set(CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH "OFF")
+    endif()
+
     if(NOT (arg_CONFIG OR arg_NO_MODULE OR arg_MODULE) AND NOT _qt_find_package_skip_find_package)
         # Try to find a config package first in quiet mode
         set(config_package_arg ${arg_UNPARSED_ARGUMENTS})
@@ -5291,6 +5297,14 @@ macro(qt_find_package)
         find_package(${arg_UNPARSED_ARGUMENTS})
     endif()
 
+    if(QT_NO_USE_FIND_PACKAGE_SYSTEM_ENVIRONMENT_PATH)
+        if("${_qt_find_package_use_system_env_backup}" STREQUAL "")
+            unset(CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH)
+        else()
+            set(CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH "${_qt_find_package_use_system_env_backup}")
+        endif()
+    endif()
+
     if(${ARGV0}_FOUND AND arg_PROVIDED_TARGETS AND NOT _qt_find_package_skip_find_package)
         # If package was found, associate each target with its package name. This will be used
         # later when creating Config files for Qt libraries, to generate correct find_dependency()