Fix EntryPointMinGW32Target.cmake to be found in top-level build

This amends commit 67dadc7e34.

We need to copy EntryPointMinGW32Target.cmake to the build dir.  The
variable config_install_dir contains path relative to the install
prefix, and the copy command needs one relative to the build dir (or an
absolute one).

Pick-to: 6.2
Fixes: QTBUG-97919
Change-Id: I4e3d735edfb69e27c248191b892422cef572258e
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/entrypoint/CMakeLists.txt

@@ -30,8 +30,10 @@ if(MINGW)
     # In prefix builds we also need to copy the file into the build config directory, so that the
     # build-dir Qt6Config.cmake finds the files when building other repos in a top-level build.
     if(QT_WILL_INSTALL)
+        get_filename_component(absolute_config_install_dir "${config_install_dir}" ABSOLUTE
+            BASE_DIR "${QT_BUILD_DIR}")
         file(COPY "${CMAKE_CURRENT_BINARY_DIR}/${mingw32target_config_file}"
-             DESTINATION "${config_install_dir}")
+             DESTINATION "${absolute_config_install_dir}")
     endif()
     include("${CMAKE_CURRENT_BINARY_DIR}/${mingw32target_config_file}")
     set(extra_cmake_includes_arg EXTRA_CMAKE_INCLUDES "${mingw32target_config_file}")