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.

main/SCsub

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 
 Import('env')
+
 from platform_methods import run_in_subprocess
 import main_builders
 
@@ -15,8 +16,6 @@ env.CommandNoCache("#main/default_controller_mappings.gen.cpp", controller_datab
 
 env.main_sources.append("#main/default_controller_mappings.gen.cpp")
 
-Export('env')
-
 env.Depends("#main/splash.gen.h", "#main/splash.png")
 env.CommandNoCache("#main/splash.gen.h", "#main/splash.png", run_in_subprocess(main_builders.make_splash))