Add and use mono build variables with cloned environment.

modules/mono/SCsub

@@ -57,10 +57,10 @@ if env['tools']:
 vars = Variables()
 vars.Add(BoolVariable('mono_glue', 'Build with the mono glue sources', True))
 vars.Add(BoolVariable('xbuild_fallback', 'If MSBuild is not found, fallback to xbuild', False))
-vars.Update(env)
+vars.Update(env_mono)
 
 # Glue sources
-if env['mono_glue']:
+if env_mono['mono_glue']:
     env_mono.add_source_files(env.modules_sources, 'glue/*.cpp')
 else:
     env_mono.Append(CPPDEFINES=['MONO_GLUE_DISABLED'])