VMS: Move defining _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED to config target

VMS: Move defining _XOPEN_SOURCE and  _XOPEN_SOURCE_EXTENDED to config target

For all other platforms that need these macros defined, that's how it's
done, so we have VMS follow suit.  That avoids a crash between in source
definitions and command line definitions on some other platforms.

Fixes #24075

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24083)