ibus: stop depending on GConf

ibus doesn't actually need gsettings-schema-convert in order to build
from tarball because it dists the needed file with the correct
timestamp.

Skip the check that would require installing gconf by hardwiring
/usr/bin/false as the path to the tool.  If by some weird reason the
tool gets used, it will fail the build (but this should not happen).

Ack'd by Javier.