Fix Ruby automake

There is a typo or error in the automake file that prevents the Ruby
library from being found or properly linked in.

Patch by "J. Ryan Earl" <oss@jryanearl.us>
See https://github.com/libgeos/libgeos/pull/32

git-svn-id: http://svn.osgeo.org/geos/trunk@3966 5242fede-7e19-0410-aef8-94bd7d2200fb