cilacc: fix openmp linking

[casperdcl]

CIL/src/Core/CMakeLists.txt

Line 26 in 5b151e1

add_definitions(${OpenMP_CXX_FLAGS})

is outdated and needs to be replaced by target_link_libraries(cliacc ... OpenMP::OpenMP_CXX) to avoid linking failures on some compilers.

FindOpenMP.cmake version/syntax might be related to #2154.

[KrisThielemans]

see also #2145 (comment) which refers to
https://github.com/conda-forge/stir-feedstock/blob/main/recipe/build.sh#L34

Though STIR does have conda-forge/stir-feedstock:recipe/meta.yaml:46-47 and funny enough UCL/STIR:src/CMakeLists.txt:137-138.

Meanwhile CIL uses TomographicImaging/CIL:recipe/meta.yaml:50-51 instead.

-- @casperdcl

I'm not sure what conda recommends these days for OpenMP libraries. I do know that the stir-feedstock builds on conda-forge :-).

Regarding the funny STIR lines, that just needs some clean-up. I've created UCL/STIR#1594. feel free to fix it :-)