ci: Test Windows + Mkvcbuild.pm in REL_16_STABLE.

ci: Test Windows + Mkvcbuild.pm in REL_16_STABLE.

* REL_15_STABLE introduced CI and tested Windows with Mkvcbuild.pm.
* REL_16_STABLE introduced Meson and switched Windows CI to that.
* REL_17_STABLE dropped Mkvcbuild.pm.

That left a blind spot when testing Makefile changes back-patched into
16.  Mkvcbuild.pm scrapes Makefiles and might break, so it's useful to
be able to check that before hitting "hamerkop" in the build farm.

Copy REL_15_STABLE's Windows task into REL_16_STABLE as a separate task,
with a few small adjustments to match later task definition style.

Discussion: https://postgr.es/m/CA%2BhUKG%2B-d0OyLMdMiZ%2BFtj2hhZXT%2B0HOyHfrPBecE_vZzh9rRg%40mail.gmail.com