Compile bullet with threasafe switch on

modules/bullet/SCsub

@@ -205,7 +205,7 @@ if env["builtin_bullet"]:
     # if env['target'] == "debug" or env['target'] == "release_debug":
     #     env_bullet.Append(CPPDEFINES=['BT_DEBUG'])
 
-    env_bullet.Append(CPPDEFINES=["BT_USE_OLD_DAMPING_METHOD"])
+    env_bullet.Append(CPPDEFINES=["BT_USE_OLD_DAMPING_METHOD", "BT_THREADSAFE"])
 
     env_thirdparty = env_bullet.Clone()
     env_thirdparty.disable_warnings()