It is useful to apply a limit to the number of processes even outside "meson test",
and specifically for clang tools. In preparation for this, generalize
determine_worker_count() to accept a variable MESON_NUM_PROCESSES instead of
MESON_TESTTHREADS, and use it throughout instead of multiprocessing.cpu_count().
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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