Do not warn if OMP_NUM_THREAD set

[llchan]

The line here https://github.com/pydata/numexpr/blob/7c2ef387d81cd450e8220fe4174cf46ec559994c/numexpr/utils.py#L118 should be

if 'NUMEXPR_MAX_THREADS' in os.environ or 'OMP_NUM_THREADS' in os.environ: 

given that the two env vars are treated the same way in the code below that. The current symptom is that this emits a spurious warning about capping num threads to 8.