Allow passing OpenMP root directory to -Dwith-openmp

[heplesser]

This PR brings the handling of -Dwith-openmp in line with MPI and other libraries according to modern CMake practice, so that the root directory of the libomp installation can be passed as an argument. In existing code, and argument different from ON or OFF is actually handled incorrectly. Also update related variable names to include lowercase letters, as is modern CMake practice, see CMP0144.

Furthermore, in the cmake summary, results for threading and MPI are now reported next to each other early in the report.