FAST_DEPEND: Workaround hacks in the wild that have '..' or '/' in OBJS.

This is mostly working around the converts/iconv port having '../ces/file.o'
in its OBJS list which resulted in '.depend../ces/file.o'. Now it will have
'.depend.._ces_file.o'.

Other implementations have :T which would result in '.depend.file.o' here, but
that could lead to collisions.

X-MFC-With: r291554
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division