x86_64-pc-windows-gnu support

[tuxmark5]

Right now skia-safe supports only x86_64-pc-windows-msvc when building for Windows. This makes it impossible to cross-compile Windows binaries from Linux that depend on skia-safe. Ideally it should also support x86_64-pc-windows-gnu, which would make it possible to cross-compile things for Windows that depend on rust-skia.

[pragmatrix]

While I appreciate such requests. For moving this forward, I would expect at least two things:

• Some basic research about what a solution would involve (for example, is it even possible to cross-compile Skia from Linux to Windows).
• A description of a use case that drives that feature request and gives the community a clear "definition of done".

I'll keep this issue open for a while to see if there is more interest in cross-compilation. Related: #285.

[pragmatrix]

I did some experiments in #346, but failed so far bringing the gnu toolchain together with clang driver & msvc Skia needs to built with.

[ctrlcctrlv]

I think this is preventing me from being able to cross compile. :(