FindOpenAL: Find AL/al.h or OpenAL/al.h in standard include paths

Add `AL` and `OpenAL` to the `PATH_SUFFIXES` so that they are searched
within the standard system include directories.

We already have `include/AL`, `include/OpenAL`, and `include` to help
out within the locations specified by `PATHS`, though it is unclear why
it was done this way instead of adding the suffixes to all the `PATHS`
explicitly.

Modules/FindOpenAL.cmake

@@ -58,7 +58,7 @@
 find_path(OPENAL_INCLUDE_DIR al.h
   HINTS
     ENV OPENALDIR
-  PATH_SUFFIXES include/AL include/OpenAL include
+  PATH_SUFFIXES include/AL include/OpenAL include AL OpenAL
   PATHS
   ~/Library/Frameworks
   /Library/Frameworks