[Appveyor] attempt to use sccache

[KrisThielemans]

No description provided.

[KrisThielemans]

This 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.

[KrisThielemans]

Stuck due to #1129.

In any case, CMake still doesn't use ccache nor sccache with the VS Generator.