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/RageFileDriverReadAhead.cpp

@@ -43,7 +43,9 @@
 #include "RageUtil.h"
 #include "RageFileManager_ReadAhead.h"
 
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 #include <cerrno>
 #if defined(WIN32)
 #include <io.h>