CMake: Simplify default CMAKE_BUILD_TYPE logic

Previously we had four-ish locations where the CMAKE_BUILD_TYPE was
force set.
Twice in QtBuildInternalsExtra.cmake via
qt_internal_force_set_cmake_build_type_conditionally(), depending on
some conditions. This was executed right at
 find_package(Qt6 COMPONENTS BuildInternals)
time.

And twice in qt_internal_set_default_build_type() via
qt_build_repo_begin() / qt_prepare_standalone_project() that goes
through QtSetup.cmake. This was executed only if the relevant functions
were called, rather than directly at find_package() time.

The exact logic of which build type ended up being set was very
confusing.

Refactor the code to decide the build type in one single location
when qt_build_repo_begin() / qt_prepare_standalone_project() are
called, rather than directly at find_package() time.

The actual logic when we override the build type depends on many
factors:
- when an explicit CMAKE_BUILD_TYPE is given, honor it, unless it's
  a multi-config build
- when it's a multi-config build, don't set any CMAKE_BUILD_TYPE,
  use the value of CMAKE_CONFIGURATION_TYPES
- when it's a qtbase build, compute a default unless an explicit value
  was given
  - the default is Debug if FEATURE_developer_build is ON
  - otherwise the default is Release
- when it's a top-level build, only choose a build type for qtbase
- when it's another repo build, use the original build type unless
  another was given explicitly (including in a top-level build)
- when it's a standalone tests build
   - if qt is multi-config, the tests will be single config, due to
     various CI failure reasons, this hasn't changed
   - if qt is single config, use the original unless an explicit
     value was given
- when it's a single standalone test build, use the original unless
  an explicit value was given

To determine when an explicit CMAKE_BUILD_TYPE was given in contrast
to when it was default initialized, we now have one single function
that uses a few heuristics.
The heuristics are needed because we can't reliably determine an
explicitly given 'Debug' build on Windows, because CMake default
initializes to that.

The heuristics include:
- checking whether CMAKE_BUILD_TYPE_INIT is different from
  CMAKE_BUILD_TYPE
- checking what the CMAKE_BUILD_TYPE was before the first project()
  call when CMake default initializes
  - we save the previous value in the qt.toolchain.cmake file
  - also in QtAutoDetect during qtbase configuration
  - also when building the sqldrivers project
- honoring the value of QT_NO_FORCE_SET_CMAKE_BUILD_TYPE

As a result of the above changes, the build type will be set exactly
zero or one times, for a particular build directory.

Note that the configure script also has some logic on which
CMAKE_BUILD_TYPE / CMAKE_CONFIGURATION_TYPES to pass to CMake
depending on whether -debug / -release / -debug-and-release /
-force-debug-info were passed. But once the values are passed,
CMake will honor them.

Amends 48841c34d2e86a741ec9992b9704c0fa5973503c
Amends 8c912cddebe544010e7da3f87af5b21f3328d7ec

Pick-to: 6.5
Fixes: QTBUG-121926
Task-number: QTBUG-114958
Task-number: QTBUG-120436
Change-Id: I30db14d1e8e9ff9bd2d7ea1d2256cdeb9493ca0d
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 49902cc6ce228c9365c54b0dbe777ae63720310c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 22ffc088e808939406346cc5f1eab94e11d8ae84)
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/plugins/sqldrivers/CMakeLists.txt b/src/plugins/sqldrivers/CMakeLists.txt
index e3254a762d..43abb00ad1 100644
--- a/src/plugins/sqldrivers/CMakeLists.txt
+++ b/src/plugins/sqldrivers/CMakeLists.txt
@@ -5,8 +5,8 @@ cmake_minimum_required(VERSION 3.16)
 if (NOT CMAKE_PROJECT_NAME STREQUAL "QtBase" AND NOT CMAKE_PROJECT_NAME STREQUAL "Qt")
     include(.cmake.conf)
     # Store initial build type (if any is specified) to be read by
-    # qt_internal_set_default_build_type().
-    # See qt_internal_set_default_build_type() for details.
+    # qt_internal_set_cmake_build_type().
+    # See qt_internal_set_cmake_build_type() for details.
     if(DEFINED CACHE{CMAKE_BUILD_TYPE})
         set(__qt_internal_standalone_project_cmake_build_type_before_project_call
             "${CMAKE_BUILD_TYPE}")