Trouble linking sdl_mixer with cmake - cmake

Question

I specifically get an undefined reference when I call Mix_OpenAudio(...). I am able to include 'SDL_mixer.h' just fine.
This is my CMakeList.txt. Sorry about all the other packages included. I'll include the FindSDL_MIXER.cmake as well.
I am able to create SDL_Mixer data types just fine as well. I installed SDL_Mixer using apt-get install.
PROJECT(Window)
CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${PROJECT_SOURCE_DIR}/CMakeModules")
add_definitions( -DMAGICKCORE_QUANTUM_DEPTH=16 )
add_definitions( -DMAGICKCORE_HDRI_ENABLE=0 )
FIND_PACKAGE(OpenGL REQUIRED)
FIND_PACKAGE(SDL2 REQUIRED)
FIND_PACKAGE(GLEW REQUIRED)
FIND_PACKAGE(GLM REQUIRED)
FIND_PACKAGE(Bullet REQUIRED)
FIND_PACKAGE(ASSIMP REQUIRED)
FIND_PACKAGE(ImageMagick COMPONENTS Magick++ REQUIRED )
FIND_PACKAGE(SDL_MIXER)
SET(CXX11_FLAGS "-std=gnu++11 -lassimp")
SET(CDEBUG_FLAGS -g)
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CXX11_FLAGS} ${CDEBUG_FLAGS}")
SET(TARGET_LIBRARIES "${OPENGL_LIBRARY} ${SDL2_LIBRARY} ${ASSIMP_LIBRARIES}")
# Find where Magick++-config lives
IF(UNIX)
ADD_DEFINITIONS(-DUNIX)
ENDIF(UNIX)
IF(NOT APPLE)
IF(GLEW_FOUND)
INCLUDE_DIRECTORIES(${GLEW_INCLUDE_DIRS})
LINK_LIBRARIES(${GLEW_LIBRARIES})
ENDIF(GLEW_FOUND)
IF(ASSIMP_FOUND)
INCLUDE_DIRECTORIES(${ASSIMP_INCLUDE_DIRS})
LINK_LIBRARIES(${ASSIMP_LIBRARIES})
ENDIF(ASSIMP_FOUND)
ENDIF(NOT APPLE)
INCLUDE_DIRECTORIES(
"${PROJECT_SOURCE_DIR}/include"
${SDL2_INCLUDE_DIR}
${GLM_INCLUDE_DIRS}
${ASSIMP_INCLUDE_DIRS}
${ImageMagick_INCLUDE_DIRS}
${BULLET_INCLUDE_DIRS}
)
# Copy shaders, models, and default config
# FILE(COPY src/shaders DESTINATION .)
# FILE(COPY models DESTINATION .)
# FILE(COPY textures DESTINATION .)
# FILE(COPY config.json DESTINATION .)
# Set Includes
SET(INCLUDES ${PROJECT_SOURCE_DIR}/include)
INCLUDE_DIRECTORIES(${INCLUDES} ${ASSIMP_INCLUDE_DIRS} ${ImageMagick_INCLUDE_DIRS} ${BULLET_INCLUDE_DIRS})
# Set sources
FILE(GLOB_RECURSE SOURCES "src/*.cpp")
ADD_EXECUTABLE(${PROJECT_NAME} ${SOURCES})
add_custom_target("${PROJECT_NAME}_SUCCESSFUL" ALL
DEPENDS ${PROJECT_NAME}
COMMAND ${CMAKE_COMMAND} -E echo ""
COMMAND ${CMAKE_COMMAND} -E echo "====================="
COMMAND ${CMAKE_COMMAND} -E echo " Compile complete!"
COMMAND ${CMAKE_COMMAND} -E echo "====================="
COMMAND ${CMAKE_COMMAND} -E echo "${CMAKE_CURRENT_BINARY_DIR}"
)
TARGET_LINK_LIBRARIES(${PROJECT_NAME} ${OPENGL_LIBRARY} ${SDL2_LIBRARY} ${ASSIMP_LIBRARY} ${ImageMagick_LIBRARIES} ${BULLET_LIBRARIES} ${SDL_MIXER_LIBRARY})
Now here is FindSDL_MIXER.cmake.
#
# Find SDL_MIXER
#
# Additional modules
include(FindPackageHandleStandardArgs)
# Find include files
find_path(
SDL_MIXER_INCLUDE_DIR
PATHS
/usr/include
/usr/local/include
/sw/include
/opt/local/include
${SDL_MIXER_ROOT_DIR}/include
DOC "The directory where SDL_mixer.h resides")
# Handle REQUIRD argument, define *_FOUND variable
#find_package_handle_standard_args(SDL_MIXER_INCLUDE_DIR)
# Hide some variables
mark_as_advanced(SDL_MIXER_INCLUDE_DIR)

Answer 1

It's because the FindSDL_MIXER.cmake isn't setting the library variable. The cmake 3.13 has an updated FindSDL_mixer.cmake that sets this correctly you can use it as an example. Specifically you need a line like this in FindSDL_MIXER.cmake to find the library and set the variable.
find_library(SDL_MIXER_LIBRARY
NAMES SDL2_mixer
HINTS
ENV SDLMIXERDIR
ENV SDLDIR
PATH_SUFFIXES lib
)
I've also noticed that SDL_MIXER_INCLUDE_DIR does not appear to be used in the CMakeLists.txt for the target. It just happens that it is probably located in the same place as some of the other header files being included.
Also you probably want to rename the file to FindSDL2_MIXER.cmake and change the corresponding FIND_PACKAGE(SDL2_MIXER). I'm not entirely sure if the posted FindSDL_MIXER.cmake is being used because it looks like the find_path is not syntactically correct, it's missing the name of the file to be found.