Port Ben's parallell Makefile hack to VMS

As far as I know, there is no MMS / MMK with parallellism today.
However, it might be added in the future (perhaps in MMK at least), so
we may as well prepare for it now.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3282)