It would be nice to have a build flag in setup.py to control whether OpenMP is used or not. There are a few cases where users might want to control this.

Currently on macOS if the system GCC is picked up, it tries to enable OpenMP, which doesn't necessarily work. Also the current build assumes that OpenMP is not supported when using Clang on macOS, but this depends on whether Apple Clang is used or some other Clang build, which does support OpenMP.

When GCC is used, then GOMP is in use, which has been shown to be not fork safe. So users may decide that they would rather not build with OpenMP support when using this compiler or when they know they plan to fork.