Enable compatibility with older GLib versions

Travis CI runs on Ubuntu 12.04, which still has GLib 2.32.

I'd be fine with requiring GLib 2.38 and saying we only support recent
versions moving forward (we still have libcangjie 1.X for older
versions), but not at the cost of our CI.

We can revisit when Travis CI upgrades their systems:
    https://github.com/travis-ci/travis-ci/issues/2046