Types of changes

Updating the openmp compiler flags on macOS, assuming libomp has bee installed using homebrew.

Motivation and context / Related issue

omp_flags returned by openmp_helpers was not correct on macOS and I got errors while building POT on Mac.

How has this been tested (if it applies)

pip install . on macOS succeeded

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.