util/postprocess-makedepend.pl: For VC, don't include system headers

All dependencies that VC gives us are absolute paths, so we need to
check if some of them are within our source or build tree.  We do that
by comparing the start of each dependency with the absolute versions
of our source and build directories.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5591)