FindBoost: Unwrap compatibility INTERFACE targets for legacy variables

Since commit 0dd6772a (FindBoost: Add legacy variables and targets for
compatibility, 2019-06-14, v3.15.0-rc2~3^2~1) we extract information
from imported targets provided by upstream `BoostConfig.cmake` files.
However, upstream Boost 1.71 provides some imported targets only for
compatibility.  They are just INTERFACE libraries that wrap around the
real target named by `INTERFACE_LINK_LIBRARIES`.  Unwrap this layer so
we can extract the real imported location.

Fixes: #19656