CMake: use target dependencies for OpenMP

[KrisThielemans]

We currently still have

STIR/src/CMakeLists.txt

Lines 137 to 147 in 4e57186

	find_package(OpenMP REQUIRED)
	add_definitions(${OpenMP_CXX_FLAGS})
	if("${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}")
	else()
	set (OpenMP_EXE_LINKER_FLAGS OpenMP::OpenMP_CXX)
	endif()
	endif()

which is largely historical. OpenMP_CXX_FLAGS doesn't seem to be used anywhere and We shouldn't need the OpenMP_EXE_LINKER_FLAGS should really be renamed to OpenMP_target_link_libraries or so, as it's only used for target_link_libraries, e.g.

STIR/src/buildblock/CMakeLists.txt

Lines 151 to 153 in 4e57186

	if (STIR_OPENMP)
	target_link_libraries(buildblock PUBLIC ${OpenMP_EXE_LINKER_FLAGS})
	endif()

STIR/src/recon_buildblock/CMakeLists.txt

Lines 148 to 150 in 4e57186

	if (STIR_OPENMP)
	target_link_libraries(recon_buildblock PUBLIC ${OpenMP_EXE_LINKER_FLAGS})
	endif()

and in the SWIG CMakeLists.txt.

I think we need the variable to work-around the atomic problem,

See also #1494