cMake txt file for linking glfw library - c++

Question

I'm learning about c++ by following a tutorial, I'm using cMake right now and I have my glfw library in my external folder but I can't get it to link correctly. Here is my cMake txt file
cmake_minimum_required(VERSION 2.8.4)
project(dunjun)
set(dunjun_src src/main.cpp)
set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin/)
set(LIBRARY_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin/)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
set(SOURCE_FILES src/main.cpp)
include_directories(
${PROJECT_SOURCE_DIR}/include
${PROJECT_SOURCE_DIR}/external/glfw-3.1/include
)
link_directories(
${PROJECT_SOURCE_DIR}/external/glfw-3.1/src
)
add_executable(dunjun ${dunjun_src})
target_link_libraries(dunjun ${PROJECT_SOURCE_DIR}/external/glfw-3.1/src)
The errors it gives me are the following.
[100%] Building CXX object CMakeFiles/dunjun.dir/src/main.cpp.o
Linking CXX executable dunjun
Undefined symbols for architecture x86_64:
"_glfwCreateWindow", referenced from:
_main in main.cpp.o
"_glfwInit", referenced from:
_main in main.cpp.o
"_glfwMakeContextCurrent", referenced from:
_main in main.cpp.o
"_glfwPollEvents", referenced from:
_main in main.cpp.o
"_glfwSwapBuffers", referenced from:
_main in main.cpp.o
"_glfwTerminate", referenced from:
_main in main.cpp.o
"_glfwWindowShouldClose", referenced from:
_main in main.cpp.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[3]: *** [dunjun] Error 1
make[2]: *** [CMakeFiles/dunjun.dir/all] Error 2
make[1]: *** [CMakeFiles/dunjun.dir/rule] Error 2
make: *** [dunjun] Error 2