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

src/modules/physics/box2d/wrap_Shape.h

@@ -42,7 +42,7 @@ namespace box2d
 	int w_Shape_computeMass(lua_State * L);
 
 	int w_Shape_destroy(lua_State * L);
-	int luaopen_shape(lua_State * L);
+	extern "C" int luaopen_shape(lua_State * L);
 
 } // box2d
 } // physics