We need to use target_include_directories so that CMake passes the right -I flags to find Taglib for the audio thumbnailer, the same way we use target_link_libraries to pass the right -L and -l flags.
On my system (with Taglib-git installed to my KF5 git root) kio-extras will find my git Taglib libraries but then try to build using the /usr/include Taglib headers. This would actually have worked OK except that my system Taglib headers are apparently old enough to be missing TagLib::MP4::File::hasMP4Tag