Fix automake compile on msys2 and mingw64

[mgreter]

This should address #172 and work with mingw64 too.
Also adds another fix from sass/libsass#2063

[mgreter]

@aroig can you check if this works on your msys2/mingw32 setup?

On another note: I didn't know about msys2 (only msys etc). I guess it would be cool to add some documentation to https://github.com/sass/libsass/blob/master/docs/build-on-windows.md for it. I pretty sure will switch myself to msys2 as I really like the pacman package manager, so kudos for that!

[aroig]

I can confirm that this builds, with both, the --host=x86_64-w64-mingw32 and the default value, which happens to be x86_64-pc-mingw64.

By the way, where does the mingw64 come from? All build scripts and toolchain packaged in msys2 use the mingw32 variant!

[aroig]

Thanks! :)

[mgreter]

I don't know when mingw32 is called mingw32 and when mingw64. Confuses me too. But if it's called mingw64 it's definitely the 64bit version. That's why I've loosened the check to only look for mingw.

[aroig]

Ah! it is script/config.guess from autotools that makes up the mingw64 name from the output of uname.

It looks like the msys2 folks use the --host=xxx-mingw32 on most of their packages as I did, possibly because of bugs like this one.

So, loosening the check to admit mingw64 seems like the right thing to do.