CMake finding packages, missing -lGLU - cmake

Question

I'm writing my first cmake file from scratch for a project that requires, amongst other things, ODE. If there are other bad practices that I'm doing, comments on that are also very welcome.
The makefile generation step with cmake is working, however in the build step with make, it complains:
Scanning dependencies of target ode_oscillex
[ 50%] Building CXX object CMakeFiles/ode_oscillex.dir/ode_oscillex.cpp.o
[100%] Linking CXX executable ode_oscillex
ld: library not found for -lGLU
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [ode_oscillex] Error 1
make[1]: *** [CMakeFiles/ode_oscillex.dir/all] Error 2
make: *** [all] Error 2
And I can't figure out what library -lGLU is supposed to be. Based on the Eclipse setup this project was previously set up in, I'm already adding X11, OpenGL and GLUT, and there shouldn't be anything else additionally. I'm setting this up on OS X.
My CMakeLists.txt is
cmake_minimum_required (VERSION 3.3)
set(PROJECT_NAME ode_oscillex)
set(TARGET ${PROJECT_NAME})
project (${PROJECT_NAME})
# Add manually built ODE library
set(ODE_PATH /Users/steve/ode-0.13.1)
set(ODE_INC_PATH ${ODE_PATH}/include)
set(ODE_LIB_PATH ${ODE_PATH}/ode/src/.libs)
set(DS_INC_PATH ${ODE_PATH}/include/drawstuff) # not sure why drawstuff needs subfolder to be specified, when ode doesn't...
set(DS_LIB_PATH ${ODE_PATH}/drawstuff/src/.libs)
include_directories(${ODE_INC_PATH} ${DS_INC_PATH})
link_directories(${ODE_LIB_PATH} ${DS_LIB_PATH})
### Add manually built Eigen
SET( EIGEN3_INCLUDE_DIR ../eigen3.1.4) # This is also sucky
include_directories(${EIGEN3_INCLUDE_DIR})
### Add Executable
add_executable(${TARGET} ${PROJECT_NAME}.cpp)
### Adding other required libraries
find_package(X11 REQUIRED)
find_package(OpenGL REQUIRED)
find_package(GLUT REQUIRED)
include_directories( ${OPENGL_INCLUDE_DIRS} ${GLUT_INCLUDE_DIRS} ${X11_INCLUDE_DIRS})
target_link_libraries(${PROJECT_NAME} ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${X11_LIBRARIES})
target_link_libraries(${TARGET} ode drawstuff GLU GL glut X11)

Answer 1

The problematic instructions for me are:
target_link_libraries(${PROJECT_NAME} ${OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${X11_LIBRARIES})
target_link_libraries(${TARGET} ode drawstuff GLU GL glut X11)
Generally, you shouldn't link "project", you need to link
"project artifacts" (i.e. "${TARGET}").
So try to rewrite this as follows:
target_link_libraries(${TARGET} <other required libraries like "ode"...> {OPENGL_LIBRARIES} ${GLUT_LIBRARIES} ${X11_LIBRARIES})
and remove the other line completely.
Generally when you called find_package()s above, those macros filled in corresponding variables which tell how to properly get proper compiler and linker flags (for OpenGL, GLUT etc respectively). So when you use appropriate variables in the target_link_libraries invocation, you can be sure that the linker flags and libraries, specific to a particular system.
On the the other hand, when you simply use target_link_libraries(<target> GLU GL glut...) you forcibly instruct cmake to link your <target> against those GL-related libraries, no matter if they installed in a particular system, nor how they are named there.
So in general you should prefer the first approach for external libraries, it's more reliable and generic.