Build improvements

This fixes non-Xcode builds on OSX, a Clang warning, and cleans up the CMake files a bit. See commit comments for details (is this a bad way to do it...?)
It's [right here](https://github.com/uppfinnarn/citra/blob/master/CMakeLists.txt#L11); `-pthread` implies `-lpthread`, and in some cases also `-D_REENTRANT` (GCC).

Explicitly linking is bad, because if you do not also specify `-pthread` to the compiler, it's possible to end up in a situation where you have pthreads available, but the C runtime is non-reentrant on certain compilers (GCC, most notably).

And, well, if you don't forget `-pthread` to the compiler stage, it's simply redundant...