[cmake,release] Provide a substitute for CMakePackageConfigHelpers.

Apparently it is not available with some older versions of CMake.
Therefore we provide a poor man's alternative for it, if it is
not there.
(cherry picked from commit 4c6240587ff1b9e7e85310a8590aa6688b270c19)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>