openmp is detected on macOS, but flags are not set

[rieder]

Describe the bug
OpenMP codes are compiled without OpenMP support on macOS

To Reproduce
Steps to reproduce the behavior:

• build AMUSE on a Mac
• install an OpenMP code (e.g. Fi)
• start a worker, e.g. with sph = Fi(mode="openmp", redirection="none")
• notice that it doesn't use OpenMP (e.g. cpu usage stays at 100% max, any OpenMP print statements aren't printed)

Expected behavior
The code uses OpenMP, and uses many cores.

Environment (please complete the following information):

• OS and version: macOS Tahoe, Conda-installed AMUSE

Additional context
Probably good to add some reporting to Fi, e.g. number of threads available?