- give precedence to ~openmp (-no-openmp) over +openmp (-openmp)
in the general rules and in the Makefile. This makes it robuster
when specifying +openmp in general, but ~openmp for specific build
components.
- disable openmp for OSspecific and Pstream components.
Neither should contain any openmp code anyhow.
Additionally, since OSspecific is generally built as a static
object, it can become problematic (eg, with AMD ROCm) if the
compiler generates information that openmp is required but then uses
static linkage.