abort on cmake config if openmp requested but not found

[gridley]

Description

We should not let CMake silently fail if CMake can't find OpenMP. Currently, the CMake flags for OpenMP can be set, but if it's not available on the system (a rarity but for Macs by default it seems!) the configuration proceeds anyway. I found it strange that this fancy Apple silicon was lagging, and of course observing the output header, no thread count info was provided!

This is an unexpected failure with a one-line fix. If a user wishes to not use OpenMP, it should be reflected in the CMake configuration flags.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Agreed — if OPENMC_USE_OPENMP is enabled, it shouldn't let it slide if OpenMP is not found. Thanks!