Add the function cmake_expand_imported_targets() to expand imported
targets in a list of libraries into their on-disk file names for a
particular configuration. Adapt the implementation from KDE's
HANDLE_IMPORTED_TARGETS_IN_CMAKE_REQUIRED_LIBRARIES which has been in
use for over 2 years. Call the function from all the Check*.cmake
macros to handle imported targets named in CMAKE_REQUIRED_LIBRARIES.
Alex