PR build/17105.

PR build/17105.

Tested with/without guile,python on amd64-linux.

I'm not sure we still have to deal with shells that can't
handle empty for lists, but I played it safe.
Otherwise this patch would be a lot smaller (though a diff -b
will still show the real changes).

	PR build/17105
	* configure.ac: Add AM_CONDITIONALs for HAVE_PYTHON, HAVE_GUILE.
	* configure: Regenerate.
	* data-directory/Makefile.in (PYTHON_FILE_LIST): Renamed from
	PYTHON_FILES.
	(PYTHON_FILES): New variable.
	(GUILE_FILE_LIST): Renamed from GUILE_FILES.
	(GUILE_FILES): New variable.
	(stamp-python, install-python, uninstall-python): Handle empty
	file list.
	(stamp-guile, install-guile, uninstall-guile): Ditto.