Compile bullet with threasafe switch on

(cherry picked from commit 6f81c213a65d288f4ae9c29472e85460dad6038b)

modules/bullet/SCsub

@@ -205,7 +205,7 @@ if env["builtin_bullet"]:
     if env["target"] == "debug" or env["target"] == "release_debug":
         env_bullet.Append(CPPDEFINES=["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()