add some missing filename expandos to extra compilers

- QMAKE_FILE_IN_EXT as an alias for QMAKE_FILE_EXT, for consistency with
  QMAKE_FILE_IN_BASE
- QMAKE_FILE_IN_NAME to make pairing _EXT/_BASE to get a full name
  unnecessary (finally ...), and make use of it
- QMAKE_FILE_OUT_PATH, because i'll need it

Change-Id: I3d91ddb84f9cce52a665d562da11d165c92550c8
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>