Trying to add SDL2_mixer and SDL2 as ExternalProject's in CMake - c

Question

I am currently trying to fetch SDL2 and SDL2_mixer as external projects in my CMake project.
SDL2 seems to work fine, but I cannot make SDL2_mixer compile. It fails when trying to link the playwav binary. The problem are the CFLAGS and LDFLAGS variables in the ExternalProject_Add. The same problem occurs when adding these variables while building from the command line without CMake.
Here is my code so far:
cmake_minimum_required(VERSION 2.8)
include(ExternalProject)
project(sdl2_test)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall")
set(LIBS_DIR ${PROJECT_BINARY_DIR}/libs)
set(SDL2_VER "2.0.8")
set(SDL2_MIXER_VER "2.0.2")
# SDL library
ExternalProject_Add(sdl2_project
URL http://www.libsdl.org/release/SDL2-${SDL2_VER}.tar.gz
PREFIX ${LIBS_DIR}/SDL2
INSTALL_COMMAND ""
)
ExternalProject_Get_Property(sdl2_project SOURCE_DIR)
ExternalProject_Get_Property(sdl2_project BINARY_DIR)
set(SDL2_SRC ${SOURCE_DIR})
set(SDL2_BIN ${BINARY_DIR})
file(GLOB SDL2_INCLUDE "${SDL2_SRC}/include/*")
file(COPY ${SDL2_INCLUDE} DESTINATION ${SDL2_BIN}/include/)
# SDL_mixer library
ExternalProject_Add(sdl2_mixer_project
URL https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-${SDL2_MIXER_VER}.tar.gz
DEPENDS sdl2_project
PREFIX ${LIBS_DIR}/SDL2_mixer
CONFIGURE_COMMAND
SDL2_CONFIG=${SDL2_BIN}/sdl2-config
CFLAGS=-I${SDL2_BIN}/include
LDFLAGS=-L${SDL2_BIN}
#LIBS=-ldl
<SOURCE_DIR>/configure
--prefix=<INSTALL_DIR>
--enable-shared=no
#--with-sdl-prefix=${SDL2_BIN}
--disable-sdltest
BUILD_COMMAND make
INSTALL_COMMAND ""
)
#file(GLOB SDL2_INCLUDE "${SDL2_SRC}/include/*")
#file(COPY ${SDL2_INCLUDE} DESTINATION ${SDL2_BIN}/include/)
ExternalProject_Get_Property(sdl2_mixer_project SOURCE_DIR)
ExternalProject_Get_Property(sdl2_mixer_project BINARY_DIR)
set(SDL2_MIXER_SRC ${SOURCE_DIR})
set(SDL2_MIXER_BIN ${BINARY_DIR})
include_directories(${SDL2_SRC}/include)
include_directories(${SDL2_MIXER_SRC}/include)
set(SOURCE sdl2test.cc)
add_executable(sdl_test ${SOURCE})
add_dependencies(sdl_test sdl2_project sdl2_mixer_project)
target_link_libraries(sdl_test ${SDL2_BIN}/libSDL2.a)
The file sdl2test.cc is just a dummy file:
#include <iostream>
int main()
{
std::cout << "Hooray" << std::endl;
return 0;
}