OSX Clang OpenMP fails on -latomic

[KrisThielemans]

To prevent problems on Linux clang, I inserted

STIR/src/CMakeLists.txt

Lines 139 to 142 in e03fbd0

	if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
	# work around https://gitlab.kitware.com/cmake/cmake/-/issues/26037
	set (OpenMP_EXE_LINKER_FLAGS OpenMP::OpenMP_CXX -latomic)
	message(STATUS "OpenMP Linker flags for Clang: ${OpenMP_EXE_LINKER_FLAGS}")

However, this gives linking problems on OSX clang on conda in conda-forge/stir-feedstock#121

ld: library not found for -latomic

Ideally we'd convert this to a link test.

See also llvm/llvm-project#73361 for the upstream problem.