ibus-cangjie does not respect --libexecdir

Created by: czchen

In debian, we want to put ibus-engine-cangjie to /usr/lib/ibus like other ibus packages. However, ibus-cangjie does not use the value set by --libexecdir to determine the path of ibus-engine-cangjie, thus we cannot change ibus-engine-cangjie easily. Please help to support --libexecdir, thanks.