CMake: Comply with include path reported by FindOpenGL.cmake

The script clearly states:

  This makes the presumption that you are include al.h like
  #include "al.h"
  and not
  #include <AL/al.h>
  The reason for this is that the latter is not entirely portable.
  Windows/Creative Labs does not by default put their headers in AL/ and
  OS X uses the convention <OpenAL/al.h>.

This commit makes default precompiled OpenAL to be properly detected
and also removes hack on MacOS which was finding the OpenAL package but
then was overwriting include directory.

Note, that new audaspace in 2.8 is using expected #include <al.h>.

intern/audaspace/OpenAL/AUD_OpenALDevice.h

@@ -37,8 +37,8 @@
 #include "AUD_Buffer.h"
 //struct AUD_OpenALBufferedFactory;
 
-#include <AL/al.h>
-#include <AL/alc.h>
+#include <al.h>
+#include <alc.h>
 #include <list>
 #include <pthread.h>