FAST_DEPEND: Only try to use dependencies from C/C++ SRCS as mkdep did.

Rather than try to guess at all of the OBJS variables just use SRCS
using the same patterns that mkdep does. This also fixes a mistake
where dependencies were being generated with FAST_DEPEND when they were
not for mkdep. This happens when OBJS!=SRCS as is the case in
gnu/lib/csu where SRCS has 1 file and OBJS has several other files that
does not even contain the 1 SRCS file. Generally in these cases the
OBJS have custom dependencies defined in their Makefile. If we generate
dependencies for those and then load a .depend file, then .IMPSRC may
contain duplicate sources and lead to errors such as:

cc: error: cannot specify -o when generating multiple output files

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division