This adds the cmQtAutoGenGlobalInitializer::GetExecutableTestOutput method
which caches the output of the called executable and returns the cached value
on any subsequent call.
This teaches CMake the variables
- CMAKE_GLOBAL_AUTOGEN_TARGET
- CMAKE_GLOBAL_AUTOGEN_TARGET_NAME
- CMAKE_GLOBAL_AUTORCC_TARGET
- CMAKE_GLOBAL_AUTORCC_TARGET_NAME
which control the generation of global
``autogen`` and ``autorcc`` targets.
Closes#17721