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

src/modules/physics/box2d/wrap_Contact.h

@@ -43,7 +43,7 @@ namespace box2d
 	int w_Contact_setEnabled(lua_State * L);
 	int w_Contact_resetFriction(lua_State * L);
 	int w_Contact_resetRestitution(lua_State * L);
-	int luaopen_contact(lua_State * L);
+	extern "C" int luaopen_contact(lua_State * L);
 
 } // box2d
 } // physics