Teach ferite.m4 on how to compare release version number

The commit

	e781263f9e03edec4797cbade1c118df4b140a25: Bump version to 1.1.18-20

Adds a release suffix "-20" to the ferite version.

The sed subtitution operations for extracting the major, minor and micro
ferite version numbers did not expect this suffix to be present so this
breaks any generated configure scripts that uses the AM_PATH_FERITE()
m4.