Fix path to aclocal in output message

datadir is defined in terms of datarootdir by default in configure
(datadir='${datarootdir}' with ${datarootdir} not expanded), so
defining datadir must happen *after* datarootdir is defined in the
glib-gettextize shell script. Otherwise ${datarootdir} is null when it
is expanded at runtime.

https://bugzilla.gnome.org/show_bug.cgi?id=518309