I couldn't reproduce the errors, so this commit is not entirely
necessary. It still works as a precaution.
The below is slightly erroneous, since the issue didn't repeat itself.
Apparently meson detects OpenMP as available on cross builds because it
can find the omp.h header, even though the library seems to be
available.
For reference, what the configure output of a native build looks like:
WARNING: OpenMP found but omp.h missing.
Run-time dependency OpenMP found: NO
3078a6a912