This is necessary because custom commands and targets may create
custom files whose names are determined by generator expressions.
For example, clang should be using $<TARGET_FILE> and $<TARGET_FILE_DIR>
instead of reverse engineering the output file name:
However, that can only be done when ADDITIONAL_MAKE_CLEAN_FILES
also accepts and evaluates generator expressions.
Similarly, KDE uses the LOCATION property where $<TARGET_FILE>
would also be better in KDE4_HANDLE_RPATH_FOR_EXECUTABLE but
also appends the result to ADDITIONAL_MAKE_CLEAN_FILES.
After this patch, both can be ported to generator expressions.