CMake, Clang and OpenMP on MacOS Big Sur - macos

Question

I'm having trouble compiling a project requiring OpenMP using CMake and clang, I set my CMakeLists.txt like this
cmake_minimum_required(VERSION 3.20)
project(cmat C)
set(CMAKE_C_STANDARD 11)
include_directories("/usr/local/include" "/usr/local/opt/llvm/include")
link_directories("/usr/local/lib" "/usr/local/opt/llvm/lib")
add_library(cmat SHARED cmat.c Calculation/_Basic_Calculate_.c Calculation/_Basic_Calculate_.h)
And when I started to build my project, it gave error
Undefined symbols for architecture x86_64:
"_omp_get_thread_num", referenced from:
_NmMulMat in _Basic_Calculate_.c.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]: *** [libcmat.dylib] Error 1
make[2]: *** [CMakeFiles/cmat.dir/all] Error 2
make[1]: *** [CMakeFiles/cmat.dir/rule] Error 2
make: *** [cmat] Error 2
I've tried some of the other answers like this and this, but they just don't work for me.
The clang version for my Mac is 13.0.0 and CMake is of 3.21.3_1, I'm using Clion as IDE, and such command will work when compiling files by shell
clang -Xpreprocessor -fopenmp -I/usr/local/include -L/usr/local/lib -lomp filename.c -o output
Great appreciate for your help.

Answer 1

You need to add -fopenmp to the linker flags as well by providing -DCMAKE_EXE_LINKER_FLAGS=-fopenmp [and any other options like -g].
In CLion (which I don't use) this would be specified in Preferences ->
Build, Execution, Deployment -> CMake -> CMake options as far as I understand the documentation.
Also please note that explicitly linking with -lomp is probably the wrong strategy, that happening should be a side-effect of specifying -fopenmp.