FAST_DEPEND: Still use if filemon is not used.

FAST_DEPEND: Still use if filemon is not used.

If filemon is used then there is no need to generate dependency files during
compilation as the .meta files will achieve the same result.

This is a temporary solution until FAST_DEPEND is default.  Once that is
default there will be an option to disable dependency generation entirely
as it is only useful if an incremental build is planned, thus META_MODE+filemon
can enable that option to short-circuit all FAST_DEPEND-related logic.

Sponsored by:	EMC / Isilon Storage Division