Allow setting OpenMP flags using env var

[thilinarmtb]

The flags to enable OpenMP depends on the compiler and it is not always
-fopenmp. This PR add the env. var OPENMP_FLAG and checks if it is set
before applying the default flag.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.28%. Comparing base (f2d9919) to head (f2c320a).
⚠️ Report is 16 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
- Coverage   97.32%   97.28%   -0.04%     
==========================================
  Files          11       11              
  Lines         224      221       -3     
==========================================
- Hits          218      215       -3     
  Misses          6        6              

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