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...?)
From IRC:

```
[19:05:51] <yuriks> is there any downside to passing -pthread on OS X?
[19:06:20] <uppfinnarn> it just makes clang mention that the argument wasn't recognized
[19:06:27] <uppfinnarn> which is kinda unnecessary, but no biggie
[19:06:44] <uppfinnarn> since pthreads are built into libc there
[19:06:48] <yuriks> I think it only does that in the linker stage: http://stackoverflow.com/questions/17841140/os-x-clang-pthread
[19:06:58] <yuriks> so you still need to pass it when compiling
[19:07:37] <uppfinnarn> ah, oops
[19:08:06] <uppfinnarn> strangely enough I didn't get any issues building or running on osx without... are we even using pthreads?
[19:08:18] <yuriks> not directly, I don't think
[19:08:24] <yuriks> we're using C++11 threads
[19:08:34] <yuriks> and GLFW and Qt are probably using pthreads internally
[19:08:47] <yuriks> I added those recently because I was getting compile errors on some configurations
[19:09:01] <uppfinnarn> ah, that means Qt automatically pulls it in properly for that target
[19:09:10] <uppfinnarn> removing the if though
[19:09:36] <uppfinnarn> actually, do we even need C flags?
[19:09:40] <uppfinnarn> are we using C anywhere?
[19:09:51] <uppfinnarn> I see it set a bit down, but it looks like copypaste
[19:10:00] <yuriks> yeah, it was GLFW+clang that was giving link errors or something
```