Add support for building on Windows via CMake.

[juj]

We should make sure that the CMake path of configuring SDL2 to build is well supported, since that is the only clean native way on Windows to build. In Windows, it might be possible to emulate configure script to work under MSYS or Cygwin, but that is not a very interesting path due to requiring the user to install such a unix environment emulation library.

[juj]

I started implementing a CMake build path, see #7 and this wiki page: https://github.com/gsathya/SDL-emscripten/wiki/Building-with-CMake

After that pull request, the CMake config goes through, but the build halts with the following log: http://pastebin.com/vN3Qfgpa

[juj]

Looks like CMake config path includes all the files in the atomics folder, most likely the configure path doesn't include those for Emscripten at all? Perhaps we should add dummy implementations of all atomics for Emscripten purposes?

[juj]

I've updated the pull request at #7 that is a fist good step configuring via CMake. It should include all the relevant bits. To be able to use #7, the pull request https://github.com/kripken/emscripten/pull/2513 needs to be merged first.

I've updated the wiki instructions in https://github.com/gsathya/SDL-emscripten/wiki/Building-with-CMake to reflect the build instructions after applying #7 and https://github.com/kripken/emscripten/pull/2513 .

@gsathya , when developing #4, can you make sure to use CMake in the test automation, so that it can be hooked up to each Windows, Linux and OSX host systems?