scanner: Downgrade non-increasing version error to warning

Commit 99a72777 introduced a new error
for when the 'since' version decreases.  It also reset the version for
messages without a version to 1.  Versioning semantics in the spec files
was a little under-specified and we don't want to break projects caught in
this grey zone.

This commits replaces previous configure.ac as the 1.4.93 tag and the
final 1.5 RC.