Audaspace: fixing problems for the merge to master suggested by Campbell and Sergey.

- rename WITH_EXTERNAL_AUDASPACE to WITH_SYSTEM_AUDASPACE.
- rename C/PYAUDASPACE to AUDASPACE_C/PY
- simplifying cmake defines and includes.
- fixing include paths and enabling WITH_SYSTEM_AUDASPACE for windows.
- fixing scons building.
- other minor build system fixes.

source/blender/python/SConscript

@@ -35,7 +35,6 @@ incs = [
     '#/intern/memutil',
     env['BF_GLEW_INC'],
     '#/intern/glew-mx',
-    '#/intern/audaspace/intern',
     '#/intern/cycles/blender',
     '../blenfont',
     '../blenkernel',