Enable Bullet DEBUG on debug builds.

modules/bullet/SCsub

@@ -200,8 +200,8 @@ if env["builtin_bullet"]:
         env_bullet.Append(CPPFLAGS=["-isystem", Dir(thirdparty_dir).path])
     else:
         env_bullet.Prepend(CPPPATH=[thirdparty_dir])
-    # if env['target'] == "debug" or env['target'] == "release_debug":
-    #     env_bullet.Append(CPPDEFINES=['BT_DEBUG'])
+    if env["target"] == "debug" or env["target"] == "release_debug":
+        env_bullet.Append(CPPDEFINES=["DEBUG"])
 
     env_thirdparty = env_bullet.Clone()
     env_thirdparty.disable_warnings()