Removing some emcmake windows-specific magic

[Sam Clegg]

If you don't use emcmake on windows you can stop reading now.

We have some logic in emcmake that is overriding the default cmake generator and trying to workaround issues with using with minggw-make on windows.  However, emscripten seems like the wrong layer to be solving these problems at so I'm proposing to remove it:

https://github.com/emscripten-core/emscripten/pull/12595

Are there any users of emcmake using the mingw-make generator who are relying on this behaviour?  

cheers,

sam