Add OpenMP thread Nb var to config script + opt. override, Refs #2622

• Checked locally that the variable OMP_NUM_THREADS is set to 1 in all config scripts and CI macros in standard setup+compile
• Checked locally that defining OMP_NUM_THREADS in the env. leads to the value being used for all config scripts and macros instead of 1

Planned reviewer for "advanced OpenMP user" review: @fweig (cannot assign due to our role model)