Allow running src/tools/msvc/mkvcbuild.pl under not Windows

Allow running src/tools/msvc/mkvcbuild.pl under not Windows

This to allow verifying the MSVC build file generation without having
to have Windows.

To do this, we avoid Windows-specific Perl modules and don't run the
"cl" compiler or "nmake".  The resulting build files won't actually be
completely correct, but it's useful enough.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/d73b2c7b-f081-8357-8422-7564d55f1aac%402ndquadrant.com