FindBoost: Enhance forward and backward compatibility

• Check if Boost_<comp>_FOUND is defined before setting it
• Fix check for Boost 1.70.0 style Boost CMakeConfig
  • Boostsystem_FOUND style was AFAIK never used
  • Boost 1.70.0 uses boost_system_FOUND and Boost_SYSTEM_FOUND
  • Legacy Boost CMake Config (according to tests) uses Boost_SYSTEM_FOUND
• Set legacy variables as defined in the Docu of FindBoost if they are not set

Open questions:

1. I'm not sure if those FPHSA calls should be done here. Looks rather to be done upstream in Boost?

• Can someone explain to me how the configs are supposed to handle messages and (set or not) the *_FOUND variable? Is a config supposed to call FPHSA?

2. @dennisklein Where did the Boostsystem_FOUND style check come from?

Fixes: #19368 (closed)
Topic-rename: FindBoost-component