Since mantaflow landed in master, clang on windows will no longer build
and crash inside clang itself. The root cause is llvm bug 43175 
I did a quick grep for #prama omp and did not see any uses of OpenMP
in mantaflow, so I think it is safe to turn this flag for clang and
sidestep this issue. But it be best if @Sebastián Barschkis (sebbas) can confirm this.