
WIN32: Support for OpenAL SDK

As anyone who uses the OpenAL SDK knows, the all
important al.h is no longer in an 'AL' directory,
thus SimGear's use of <AL/al.h> is a problem.

I propose a switch - #ifdef USE_OPENAL_SDK
and uder this switch, only include <al.h>.

This effects four(4) files, but in three cases
the #include <AL/al.h> is redundant.

The four(4) files are :
sample_openal.cxx
sample_openal.hxx
soundmgr_openal.cxx
soundmgr_openal.hxx

However, since the two(2) cxx files, include their
repective headers, then the <AL/al.h> blocks can be
completely removed from these two files.

Further, soundmgr_openal.hxx also includes sample_openal.hxx,
then again the <AL/al.h> block could be removed from
sample_openal.hxx.

So the final modified block is only in sample_openal.hxx,
and it is -

#if defined(__APPLE__)
# define AL_ILLEGAL_ENUM AL_INVALID_ENUM
# define AL_ILLEGAL_COMMAND AL_INVALID_OPERATION
# include <OpenAL/al.h>
# include <OpenAL/alut.h>
#else
#ifdef USE_OPENAL_SDK
# include <al.h>
#else
# include <AL/al.h>
#endif
# include <AL/alut.h>
#endif

As is now usual, I have also tried this change in
Ubuntu, and SG/FG compile fine with this change.

