build: Detect MSYS2 builds on Windows

Otherwise, when building inside an MSYS2 environment the required
`-no-undefined` linker flag won't be used to build Graphene.
docs: Mention GLib in the MSYS2 instructions

If GLib is not installed, Graphene will still automatically turn off
functionality, but if you're building a GLib-based application on
Windows you probably want to ensure you build GLib before Graphene.