Use CMakeLists.txt instead of a custom Makefile

[devnoname120]

For some reason this library is compiled using a Makefile.vita file. There are side effects to this such as sdl2.pc not being created: https://github.com/rsn8887/SDL-Vita/blob/master/CMakeLists.txt#L1639

This is an issue for programs such as OpenRCT2 that use pkgconfig to find libraries.