chore(windows): move from Borland make to nmake

While it may be smarter to move to a different build system altogether,
it is also a *lot* more work. This was a fairly minor syntax refresh to
get nmake working with our existing Makefile system. This is now no
longer compatible with Borland make, so you will need to use `nmake` to
build.

The presence of NMAKE.md informs builds systems that nmake should be
used instead of make.