FindBoost: always define BOOST_ALL_DYN_LINK for Boost::dynamic_linking
The purpose of the
Boost::dynamic_linking interface library is to cause
BOOST_ALL_DYN_LINK to be defined. Do this on all platforms instead of just Windows.
In particular, using
Boost::log trivial_logger requires to set
Boost::Log is compiled as dynamic library.
Fixes: #17813 (closed)