wcs* fix-up for configure.com

VMS has had wcscmp and wcsxfrm since version 6.2 was released in
1995.  None of these functions depends on what compiler we're
running, only on what the C library has, and there is only one
viable CRTL in existence in modern times, so we probably need a
further refactor to eliminate the compiler check for this whole
section.
A malloc cast in vms/vms.c.

C didn't notice this with standard warnings enabled, but C++ takes
vengeance, so cast appropriately.
Move a VMS-specific include file in sv.c.

C++ doesn't allow pulling in rms.h smack dab in the middle of a
function like that.  While we're in there, go ahead and start
using the correct __VMS macro instead of just VMS.  The latter
was only necessary for VAX C, the non-C89-compliant predecessor
to DEC C that died a natural death some years ago.