Minor fixes for macOS/Clang and use as a subproject

[drbenmorgan]

Just a few small fixes found when using PTL in Geant4:

• Address compile issues on macOS/Clang with missing header and initialisation order
• Backport fix to TaskRunManager::Terminate checking for non-null thread pool before accessing it
• Silence output to cerr in ThreadPool::destroy_threadpool for zero verbosity
  • This caused test failures in Geant4 as CTest regards stderr output as a fail
• Only enable formatting for a master project, otherwise options are propagated to parent project
• Bump "maximum" CMake version to 3.23 (tested locally, no policy issues identified)