Bump ferite version to 1.1.18.21

Teach ferite.m4 to handle both "-" and "." in release number separator.

Without this change, dist tarballs whose "configure" file has been
configured with pre-1.1.80-20 AM_PATH_FERITE would fail when comparing
their wanted ferite version with ferite version 1.1.80-20.

The commit

  e781263f9e03edec4797cbade1c118df4b140a25 (tag: 1.1.18-20)
  Author: Nazri Ramliy <nazri_ramliy@cention.asia>
  Date:   2014-08-15 16:03:47 +0800

      Bump version to 1.1.18-20

      The release in ferite.spec is 19, hence the bump to 20 here

conflates the ferite version with the rpm release number into ferite
version. This is bad.

The ferite version should have been bumped to 1.1.18.1 instead of
1.1.18-20. But we can't retroactively fix that now since the tag
1.1.18-20 is already pushed.