Use proper cmake detection of features.

This attempts to organize all needed items and places appropriate defines
in a single location. Redundant comparisons/defines were removed when noticed.

A few caveats, however:

* This may be better targeted towards the 5_1_0 branch instead of master right now.
* Cmake will run a little slower on configuration/generation now. That's due to the sanity checks it runs.
* There are some more checks to be added later, but this should be a solid start.

Thanks to the [libical](https://github.com/libical/libical) project for inspiration.

src/RageSoundReader_ThreadedBuffer.cpp

@@ -5,10 +5,6 @@
 #include "Foreach.h"
 #include "RageLog.h"
 
-#if !defined(_WINDOWS)
-#include <unistd.h>
-#endif
-
 /* Implement threaded read-ahead buffering.
  *
  * If a buffer is low on data, keep filling until it has a g_iMinFillFrames.