FindBoost: Tolerate future Boost INTERFACE libraries

Since commit 0dd6772a (FindBoost: Add legacy variables and targets for
compatibility, 2019-06-14, v3.15.0-rc2~3^2~1) 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: Alexander Grund <alexander.grund@tu-dresden.de>
Issue: #19656, #19402