Build fails when configuring twice with CMake and with both INSTALL_MKSPECSDIR and QT_QMAKE_TARGET_MKSPEC set

Issue:

When both the "INSTALL_MKSPECSDIR" and "QT_QMAKE_TARGET_MKSPEC" variables are provided, building the Qt fails if it has been configured more than two times. This probably impacts previous versions too.

Reproduction: 

• Run this twice:

cmake {qt5_source_dir} -G Ninja \
  -DQT_QMAKE_TARGET_MKSPEC=linux-g++ \
  -DINSTALL_MKSPECSDIR=install/archdatadir/mkspecs

• Build:

cmake --build . -v -j

• Compiling every translation unit fails with:

/home/tibo/Downloads/qt5/qtbase/src/corelib/io/qfilesystemmetadata_p.h:54:10: fatal error: qplatformdefs.h: No such file or directory
   54 | #include "qplatformdefs.h"

Causes:

The variable "
QT_DEFAULT_PLATFORM_DEFINITION_DIR_ABSOLUTE" is defined differently in "QtBuild.cmake" depending on whether "QT_PLATFORM_DEFINITION_DIR" is defined or not. The second time the project is configured, "QT_PLATFORM_DEFINITION_DIR" is is the cache (and defined) and the "QT_DEFAULT_PLATFORM_DEFINITION_DIR_ABSOLUTE" is set to a non-existing directory.