Use imported target Boost::headers
Available since CMake 3.15.
Motivated by having currently not having ${Boost_INCLUDE_DIRS} used as SYSTEM include dirs, which resulted in build failing over some undefined macro in the boost headers due to Werror=undef as now set with KDE_COMPILERSETTINGS_LEVEL 5.96. So while at it, switching to imported target for modern CMake code.
@hindenburg Can you tell if this works for your macOS build as well? (should)
Edited by Friedrich W. H. Kossebau