build: Detect MSYS2 builds on Windows
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.

criptych/graphene,criptych/graphene,criptych/graphene,criptych/graphene,ebassi/graphene,ebassi/graphene