Use rel2abs() on VMS, rather than realpath()

It seems realpath() is quite buggy on VMS, or will at least give quite
surprising results.  On the other hand, realpath() is the better on
Unix to clean out clutter like foo/../bar on Unix.

So we make out own function to get the absolute directory for a given
input, and use rel2abs() or realpath() depending on the platform
Configure runs on.

Issue reported by Steven M. Schweda <sms@antinode.info>

Reviewed-by: Andy Polyakov <appro@openssl.org>