Connect lib/libomp to the build.
- Set MK_OPENMP to yes by default only on amd64, for now.
- Bump __FreeBSD_version to signal this addition.
- Ensure gcc's conflicting omp.h is not installed if MK_OPENMP is yes.
- Update OptionalObsoleteFiles.inc to cope with the conflicting omp.h.
- Regenerate src.conf(5) with new WITH/WITHOUT fragments.
Relnotes: yes
PR: 236062
MFC after: 1 month
X-MFC-With: r344779