SCons: Better validation for platform-specific opt-in drivers

This replaces cryptic compilation errors with a clear error message
and early build termination.

platform/windows/detect.py

@@ -247,7 +247,7 @@ def get_flags():
 
     return {
         "arch": arch,
-        "supported": ["mono"],
+        "supported": ["d3d12", "mono", "xaudio2"],
     }