FindBoost: Don't overwrite Boost_<comp>_FOUND
The current FindBoost unconditionally overwrites Boost_<comp>_FOUND
: https://gitlab.kitware.com/cmake/cmake/blob/v3.15.0-rc1/Modules/FindBoost.cmake#L286
This makes it impossible for the future boost to handle that variable properly by defining it itself.
Solution: Guard with if(DEFINED