FindBoost: Tolerate future Boost INTERFACE libraries
Since !3438 (merged) we query imported targets
provided by BoostConfig.cmake
for their IMPORTED_LOCATION_<cfg>
.
Querying this property is not allowed on INTERFACE libraries, so add a
condition to avoid doing so in case Boost adds one in the future.
Suggested-by: @Flamefire
Issue: #19656 (closed), #19402
Edited by Brad King