improve control for openmp, and ccache

Followup on various exaFOAM discussions

• it would be useful to add tighter openmp integration. However, cannot be certain that it is installed on all systems.
• additional hooks for ccache, or equivalent. This would allow lifting of extra instrumentation (eg, TU-Darmstadt)

@sbna @skreutzer

assigned to @mark

Thoughts:

• for openmp, can add in a +openmp compile control that would add in COMP_OPENMP and LINK_OPENMP in the appropriate places.
• for ccache, can add in a +ccache compile control, but also support an extended syntax (eg, "ccache=<ccache -options>") that would allow other mechanisms.

mentioned in commit 9f7cfa94

changed the description

closed