Very long SRCS list with unusually long src/ prefix

Gang,

Attached is the patch for bsd.dep.mk that addresses the problem
of "make depend" hitting the E2BIG limit when running mkdep(1)
with a very long argument list, caused by very long SRCS list
(as in libc), and with unusually long src/ prefix, that some of
you have experienced in the past.

Demo:

: SRCS!=	jot 50000
: all:
: 	_at_echo mkdep ${SRCS}

We already use an approach similar to what is in the patch, to
build kernel dependencies, except here we also need to take into
account possible .PATH: hints, hence some complexities in the
implementation.

Comments/reviews/suggestions are highly accepted and appreciated.

(Don't suggest an approach used by NetBSD, by running mkdep(1)
for each source file separately.  It has been attempted and is
deemed inefficient.)


Cheers,
-- 
Ruslan Ermilov
FreeBSD committer
ru_at_FreeBSD.org