mozjs should be able to build with system-installed MSYS2 on Windows

[TheGoddessInari]

I've been trying to find someone to talk to about the moz-buildtools using extremely outdated MSYS1. MSYS1 won't work (at all) on machines with ALSR enabled, while MSYS2 can work with some PE flags (which I've also asked MSYS2 people to use by default since it's worked reasonably ever since MSYS2 came out).

This hard dependency on moz_buildtools and MSYS1 (among other things) makes building on Windows next to impossible if you have an existing environment setup, especially if you have ALSR on.

Since nobody has responded on mozilla IRC channels about this in months, let's just cut to the chase and let mozjs work with a system installation of MSYS2!

Like with servo, there doesn't seem to be any particular reason to explicitly depend on MOZ_BUILDTOOLS, a special python setup, and a special environment if the user can reasonably provide a working one themselves. All it does is make it more difficult for third parties (like myself) to get a working servo embedding.

All it would take is a little cooperation with users able to provide their own (full) build system and command-line environment cleanly. This isn't that hard on Windows, although Microsoft doesn't provide effective documentation on this, even though they provide resources like vcpkg.