COMP: stricter handling of openmp vs no-openmp

- 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.

src/parallel/decompose/Allwmake-mpi

@@ -15,7 +15,8 @@ if have_scotch
 then
     if have_ptscotch
     then
-        wmakeLibMpi ptscotchDecomp "scotch=$SCOTCH_VERSION"
+        # Never need/want openmp for MPI interfaces
+        wmakeLibMpi -no-openmp ptscotchDecomp "scotch=$SCOTCH_VERSION"
     fi
 fi