It seems enchant1 is unmaintained and all the active development went to enchant2 some time ago. It would be good to switch gtkspell3 to use enchant2 too. Attached is a simple patch which does just that. If you'd like to have a choice between enchant1 and enchant2, then it would require more autotools foo with conditional compilation.
Thanks a lot, I actually made these exact changes a while ago but never ended up doing a release - did so now!
This helps a lot as the new enchant library fixes some issues in relocability.