cmake: Only config Boost during find_package

Without the CONFIG option, find_package will perform Module search. On
at least Linux Mint 20 (I'm unable to reproduce this on CentOS and Arch
Linux), my guess is that this causes CMake to find "dirty" modules that
modify the configuration state despite the Boost version being too
low/absent.

Use CONFIG to put CMake into pure Config mode and avoid Module search.