gschem: Fix duplicate keybindings.

Fix duplicate keybindings for Edit->Pin Type and Edit->Update.

Closes-bug: lp-1255064
build-sys: Add explicit check for GIO.

Although GIO is distributed as part of the GLib source tree and is
also a dependency of GTK, sometimes it seems like the headers don't
get installed.  It's useful to give the user some info about this.

Affects-bug: lp-1258217