CMake: Enforce minimum CMake version in user projects

This change introduces new behavior to error out when configuring user
projects if the CMake version used is too old for Qt to work with.

The main motivator is the requirement of new CMake features to ensure
object libraries are placed in the proper place on the link line in
static builds.

The minimum CMake version is computed based on whether Qt was
configured as shared or static libraries.

At the moment the required versions for building and using Qt are the
same.

The minimum versions are defined in qtbase/.cmake.conf in the
following variables

QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_BUILDING_QT_SHARED
QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_BUILDING_QT_STATIC
QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_SHARED
QT_SUPPORTED_MIN_CMAKE_VERSION_FOR_USING_QT_STATIC

Qt Packagers can disable the version check when configuring Qt
by setting
QT_FORCE_MIN_CMAKE_VERSION_FOR_BUILDING_QT and
QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT.

In this case it is the packagers responsibility to ensure such a Qt
works correctly with the specified CMake version.

User projects can also set QT_FORCE_MIN_CMAKE_VERSION_FOR_USING_QT
to disable the version check. Then it's the project's developer
responsibility to ensure such a Qt works correctly.

No official support is provided for these cases.

Implementation notes.

The versions required to build Qt are stored in
QtBuildInternalsExtra.cmake
whereas the versions required to use Qt are stored in a new
QtConfigExtras.cmake.

Also the policy range variables stored in
QtBuildInternalsExtra.cmake are now regular variables instead of cache
variables, to properly allow overrides per-repository.

Some renaming of functions and variables was done for a bit more
clarity and easier grep-ability.

Pick-to: 6.2
Task-number: QTBUG-95018
Change-Id: I4279f2e10b6d3977319237ba21e2f4ed676aa48b
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 10 insertions, 12 deletions

diff --git a/cmake/QtPostProcessHelpers.cmake b/cmake/QtPostProcessHelpers.cmake
index 248989c81c..f8f42d273e 100644
--- a/cmake/QtPostProcessHelpers.cmake
+++ b/cmake/QtPostProcessHelpers.cmake
@@ -556,18 +556,16 @@ endif()\n")
         endif()
 
         # Save minimum and policy-related CMake versions to ensure the same minimum is
-        # checked for when building other child repos (qtsvg, etc) and the policy settings
-        # will be consistent unless the child repos explicitly override them.
-        qt_internal_get_qt_supported_minimum_cmake_version(min_supported_version)
-        qt_internal_get_computed_minimum_cmake_version(computed_min_version)
-        qt_internal_get_min_new_policy_cmake_version(lower_policy_version)
-        qt_internal_get_max_new_policy_cmake_version(upper_policy_version)
-        string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS
-            "set(QT_MIN_SUPPORTED_CMAKE_VERSION \"${min_supported_version}\" CACHE STRING \"Minimum supported CMake version required to build Qt\")\n"
-            "set(QT_COMPUTED_MIN_CMAKE_VERSION \"${computed_min_version}\" CACHE STRING \"Computed minimum CMake version required to build Qt\")\n"
-            "set(QT_MIN_NEW_POLICY_CMAKE_VERSION \"${lower_policy_version}\" CACHE STRING \"Oldest CMake version for which NEW policies should be enabled\")\n"
-            "set(QT_MAX_NEW_POLICY_CMAKE_VERSION \"${upper_policy_version}\" CACHE STRING \"Latest CMake version for which NEW policies should be enabled\")\n"
-        )
+        # checked for when building other downstream repos (qtsvg, etc) and the policy settings
+        # will be consistent unless the downstream repos explicitly override them.
+        # Policy settings can be overridden per-repo, but the minimum CMake version is global for all of
+        # Qt.
+        qt_internal_get_supported_min_cmake_version_for_building_qt(
+            supported_min_version_for_building_qt)
+        qt_internal_get_computed_min_cmake_version_for_building_qt(
+            computed_min_version_for_building_qt)
+        qt_internal_get_min_new_policy_cmake_version(min_new_policy_version)
+        qt_internal_get_max_new_policy_cmake_version(max_new_policy_version)
 
         # Rpath related things that need to be re-used when building other repos.
         string(APPEND QT_EXTRA_BUILD_INTERNALS_VARS