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

src/modules/physics/box2d/wrap_CircleShape.h

@@ -35,7 +35,7 @@ namespace box2d
 	CircleShape * luax_checkcircleshape(lua_State * L, int idx);
 	int w_CircleShape_getRadius(lua_State * L);
 	int w_CircleShape_setRadius(lua_State * L);
-	int luaopen_circleshape(lua_State * L);
+	extern "C" int luaopen_circleshape(lua_State * L);
 
 } // box2d
 } // physics