FAST_DEPEND: Fix building of wrong source files in some cases.

FAST_DEPEND: Fix building of wrong source files in some cases.

Similar to the original reason for these dependency hints to be added,
in r124637, the missing-dependency file case can lead to building of the
wrong source.

A clear example of this is in gnu/lib/libstdc++ where the .PATH contains
both contrib/gcc and contrib/libstdc++/src.

  contrib/gcc has a debug.c.
  contrib/libstdc++/src has a debug.cc.

  When building for the objects of debug.o, debug.So, and debug.po, it is
  ambiguous for which src file to use due to the suffix transformation
  rules, even though the proper one is listed first in .PATH.

  This was normally avoided due to these dependency hints for the initial
  build, and then mkdep would add an explicit 'debug.o: debug.cc'
  dependency into the .depend file.  WITH_FAST_DEPEND does not generate
  the .depend file with these, but puts them into .depend.debug.o instead.

Rather than extending the exists() check to each object's .depend.*
file, just enable the hint when when using WITH_FAST_DEPEND.  It fixes
the problem and seems to be safe enough to use since it is mapping SRCS
back to OBJS, rather than letting make make assumptions from OBJS to
SRCS.

A similar check mapping objects to headers is present in some mk files
but was not extended here for FAST_DEPEND since it has not yet been
found to be a problem.

X-MFC-With:	r290433
MFC after:	3 weeks
Sponsored by:	EMC / Isilon Storage Division