Windows support

[fmang]

There's no reason oshu! wouldn't work on Windows, but someone has to build it.

A few things probably won't work right away, like getopt or possibly some POSIX function calls here and there.

1. Tweak the code until it builds.
2. Port the desktop integrations.
3. Make an installer.

[jamuwu]

how much work do you think would go into making oshu use cmake to compile instead of autoconf (I think it's autoconf at least)?

[fmang]

oshu!'s current build system is pretty much as simple as it could get. Someone familiar would port it in the blink of an eye.

My main concern are the dependencies: ffmpeg, SDL2, cairo, pango. I have no clue how these things integrate in a Windows build environment.