MSVC build improvements

Hi, this pull has a couple of compile fixes for MSVC x86 builds. I have some further patches for x64 builds but they still need some work so I propose we deal with those in another pull.

I added an alternative build system based on Premake that should make it a bit easier to mantain the builds on Windows. Specifically it can generate projects for different versions of VS and should be much easier to extend in the future since it's very easily scripted in Lua.

It still does not support all of the targets that the existing projects have (tests, profilers and some tools) and the config support could be improved too, but those can easily be added if you agree this is a direction we should take for Windows support.