Make version check of suggested modules non-fatal.

Previously if a suggested module was found but it had a wrong version Cmake did emit a fatal error. With this commit we only display a warning if the version does not match for a suggested module.

This is the first part of a fix needed for dune-grid#72 (closed)

mentioned in merge request dune-grid!263 (merged)

merged

mentioned in commit 601c6f8e

mentioned in commit d72f5e3c

mentioned in merge request !443 (merged)

mentioned in merge request !453 (merged)