FindBoost: Tolerate future Boost INTERFACE libraries

Since commit 0dd6772a89 (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 <[email protected]>
Issue: #19656, #19402

Modules/FindBoost.cmake

@@ -353,14 +353,20 @@ function(_boost_set_legacy_variables_from_config)
       _boost_set_if_unset(Boost_${uppercomp}_LIBRARY "${target}")
       _boost_set_if_unset(Boost_${uppercomp}_LIBRARIES "${target}") # Very old legacy variable
       list(APPEND libraries "${target}")
-      foreach(cfg RELEASE DEBUG)
-        get_target_property(lib ${target} IMPORTED_LOCATION_${cfg})
-        if(lib)
-          get_filename_component(lib_dir "${lib}" DIRECTORY)
-          list(APPEND library_dirs ${lib_dir})
-          _boost_set_cache_if_unset(Boost_${uppercomp}_LIBRARY_${cfg} "${lib}")
-        endif()
-      endforeach()
+      get_property(type TARGET "${target}" PROPERTY TYPE)
+      if(NOT type STREQUAL "INTERFACE_LIBRARY")
+        foreach(cfg RELEASE DEBUG)
+          get_target_property(lib ${target} IMPORTED_LOCATION_${cfg})
+          if(lib)
+            get_filename_component(lib_dir "${lib}" DIRECTORY)
+            list(APPEND library_dirs ${lib_dir})
+            _boost_set_cache_if_unset(Boost_${uppercomp}_LIBRARY_${cfg} "${lib}")
+          endif()
+        endforeach()
+      elseif(Boost_DEBUG OR Boost_VERBOSE)
+        # For projects using only the Boost::* targets this warning can be safely ignored.
+        message(WARNING "Imported target '${target}' for required component '${comp}' has no artifact. Legacy variables for this component might be missing. Refer to the documentation of your Boost installation for help on variables to use.")
+      endif()
       _boost_get_canonical_target_name("${comp}" canonical_target)
       if(NOT TARGET "${canonical_target}")
         add_library("${canonical_target}" INTERFACE IMPORTED)