QMAKE_[DIST]CLEAN doesn't work with directories

All items specified with QMAKE_CLEAN and QMAKE_DISTCLEAN in the project file are treated as files.
Specifying directories causes errors when "make [dist]clean" is called.
Instead, qmake should detect directories and thread them properly.
As an alternative, new variables (e.g. QMAKE_[DIST]CLEAN_DIR) could be added for specifying directories.