Properly export all lua-facing functions and use those for initialization (love.cpp no longer explicitly mentions implementations)

src/modules/physics/box2d/wrap_Fixture.h

@@ -58,7 +58,7 @@ namespace box2d
 	int w_Fixture_getGroupIndex(lua_State * L);
 	int w_Fixture_setGroupIndex(lua_State * L);
 	int w_Fixture_destroy(lua_State * L);
-	int luaopen_fixture(lua_State * L);
+	extern "C" int luaopen_fixture(lua_State * L);
 
 } // box2d
 } // physics