ExternalProject learned how to deal with that using DOWNLOAD_EXTRACT_TIMESTAMP OFF if you want to use that to download (and extract) things for you.
Also note that one wants to continue using cached values in case, say, a new version is installed somewhere in a search path. Otherwise it becomes very difficult to “stick” to something not found first in the search path because it is hard to know if the results come from actually wanting what has already been found or whether something “new” should be preferred.