Drop the hardcoded gettext package

Created by: bochecha

One possibility would be to have the ibus-engine-cangjie script get it from the Makefile.am and then pass it as a parameter.

Not very nice, but that script is already generated at build-time, so it would be better than making src/config.py generated as well. (I used to have it that way, and it was annoying)

Can't think about a better way right now, but any idea is welcome. :)