Policy `CMP0074` was added by commit eb35d8884b (find_package: Use
PackageName_ROOT variables as search prefixes, 2018-03-15). Revise the
logic to avoid warning when a `PackageName_ROOT` variable is set to
empty since that won't change the search behavior. Also, when we do
warn include the variable value(s) for reference.
Brad King
3f479a3ca3