Visual Studio 2013 and Windows SDK 8.1 not supported + cross-compilers used by default

I can create a pull request for the 2k13/8.1 support. It might need some deeper refactoring in order to handle the whole x86/amd64/arm platforms properly across the whole toolchain, but the detection part is simple.

There is also an issue (I think) with the cl.exe used for builds: for x64 and ia64, the cross-compilers are used rather than the “normal” ones (x86_amd64 instead of amd64). If that’s indeed a bug I can include that fix in the pull request…

Well, just in case: