FAST_DEPEND: Fix 'make all install' not properly rebuilding based on .depend.* files.

FAST_DEPEND: Fix 'make all install' not properly rebuilding based on .depend.* files.

An optimization is in place to skip reading the .depend.* files with
'make install'.  This was too strong and broke 'make all install' and
'make foo.o foo install'.  Now only skip reading the dependency files
if all make targets ran are install targets.

The problem comes about because headers are only added in as a guessed
dependency if .depend.* files do not yet exist.  If they do exist, even
if being skipped from being read, then the header dependencies are not
applied.  This applies to all #included files, and not just headers.

Reported by:	kib
MFC after:	1 day
Sponsored by:	EMC / Isilon Storage Division