make OpenMP an optional dependency

[njzjz]

Fix #1491.

[codecov-commenter]

Codecov Report

Merging #1498 (231d8dd) into devel (d9a4a86) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel    #1498   +/-   ##
=======================================
  Coverage   76.03%   76.03%           
=======================================
  Files          93       93           
  Lines        7729     7729           
=======================================
  Hits         5877     5877           
  Misses       1852     1852           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9a4a86...231d8dd. Read the comment docs.