OpenMP linking errors on Windows using Ninja and clang

#1127 attempts to use Ninja on Appveyor (among trying to use sccache). It fails due to (unrelated) linking errors with OpenMP. Apparently the linker doesn't get passed the correct flag. This is likely caused by switching to Ninja, which I had to do as the VS Generator ignores the *COMPILER_LAUNCHER (I guess I could try nmake). But this is using the clang compiler as opposed to cl. so lots of things change.

errors like

lld-link: error: undefined symbol: __kmpc_dispatch_init_8
>>> referenced by iterative_KOSMAPOSL.lib(KOSMAPOSLReconstruction.cxx.obj):(.omp_outlined.)
>>> referenced by iterative_KOSMAPOSL.lib(KOSMAPOSLReconstruction.cxx.obj):(.omp_outlined..1)

and

lld-link: error: undefined symbol: omp_get_num_threads
>>> referenced by recon_buildblock.lib(PoissonLogLikelihoodWithLinearModelForMeanAndProjData.cxx.obj):(.omp_outlined.)
>>> referenced by recon_buildblock.lib(PoissonLogLikelihoodWithLinearModelForMeanAndProjData.cxx.obj):(.omp_outlined..1)

I don't think this is our problem, but CMake's FindOpenMP.

Originally posted by @KrisThielemans in https://github.com/UCL/STIR/issues/1127#issuecomment-1374934263

Start with the AppVeyor build from #1127 and the CMake FindOpenMP configuration used with Ninja and clang on Windows. Reproduce the linker errors for __kmpc_dispatch_init_8 and omp_get_num_threads, then confirm that a corrected configuration links the OpenMP symbols successfully.