cmComputeTargetDepends: Use valid config to compute target depends.
If CMAKE_BUILD_TYPE is set, and user code contains: target_link_libraries(myexe prefix_$<$<CONFIG:Debug>:debug>) then the computation with an empty config was computing a target-level dependency on a target or library called prefix_, and a dependency on a target or library called prefix_debug (as expected). The existing logic skips 'prefix_' because it is not a known target, and defers to the link-dependencies logic to find the library. The link-dependencies logic does not incorrectly handle the config as cmComputeTargetDepends did, and so did not encounter 'prefix_' during its computation. This likely had no effect on the generated buildsystem.