体系结构x86_64的未定义符号，链接器命令失败

我刚开始学习 c++ 和 makefiles。现在我被困住了。像这样的问题似乎有几十个，但我无法弄清楚哪个答案适用于我的情况。 这个答案大概是显而易见的。一些关于在哪里寻找的线索和我所做的事情是错误的原因将不胜感激！

这是我的错误：

Undefined symbols for architecture x86_64:
  "App::init(char const*, int, int, int, int, bool)", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [../out/MYAPP] Error 1

这是我的代码：

主.cpp

#include "App.h"
// our App object
App* a_app = 0;
int main(int argc, char* argv[])
{
    a_app = new App();
    a_app->init("Hello World", 100, 100, 640, 480, true);
    //...etc more code
    return 0;
}

应用程式

#ifndef __App__
#define __App__
#include <SDL2/SDL.h>
class App
{
public:
    App(){}
    ~App(){}
    bool init(const char* title, int xpos, int ypos, int width, int height, bool fullscreen);
    //..etc
private:
};
#endif /* defined(__App__) */

应用.cpp

#include "App.h"
#include <iostream>
using namespace std;
bool App::init(const char* title, int xpos, int ypos, int width, int height, bool fullscreen)
{   
    // attempt to initialize SDL
    if(SDL_Init(SDL_INIT_EVERYTHING) == 0)
    {
        cout << "SDL init success n";
        int flags = 0;
        if(fullscreen)
        {
            flags = SDL_WINDOW_FULLSCREEN;
        }
        // init the window
        m_pWindow = SDL_CreateWindow(title, xpos, ypos, width, height, flags);
            //...etc 
    }
    else
    {
        cout << "SDL init fail n";
        return false; // SDL init fail
    }
    cout << "SDL init success n";
    m_bRunning = true; // everything inited successfully, start the main loop
    return true;
}

最后是我的制作文件

CXX = clang++
SDL = -framework SDL2 -framework SDL2_image
INCLUDES = -I ~/Library/Frameworks/SDL2.framework/Headers -I ~/Library/Frameworks/SDL2_image.framework/Headers
CXXFLAGS = -Wall -c -std=c++11 $(INCLUDES)
LDFLAGS = $(SDL) -F ~/Library/Frameworks/
SRC = src
BUILD = build
OUT = ../out
EXE = $(OUT)/MYAPP
OBJECTS = $(BUILD)/main.o $(BUILD)/App.o
all: $(EXE)
$(EXE): $(OBJECTS) | $(OUT)
    $(CXX) $(LDFLAGS) $< -o $@
$(BUILD)/%.o : $(SRC)/%.cpp | $(BUILD)
    $(CXX) $(CXXFLAGS) $< -o $@ 
$(BUILD):
    mkdir -p $(BUILD)
$(OUT):
    mkdir -p $(OUT)
clean:
    rm $(BUILD)/*.o && rm $(EXE)