SCons: Build thirdparty code in own env, disable warnings

Also remove unnecessary `Export('env')` in other SCsubs,
Export should only be used when exporting *new* objects.

drivers/xaudio2/SCsub

@@ -5,5 +5,3 @@ Import('env')
 env.add_source_files(env.drivers_sources, "*.cpp")
 env.Append(CXXFLAGS=['-DXAUDIO2_ENABLED'])
 env.Append(LINKFLAGS=['xaudio2_8.lib'])
-
-Export('env')