configure.ac.in: fix missing XDT_I18N call

glib's gettext machinery wasn't set up correctly, on make run getting the error:

/usr/bin/msgfmt: error while opening "@MSGFMT_OPTS@" for reading: No such file or directory

In xfce projects, this is solved by calling the XDT_I18N macro.

Closes: metux/libxfce4windowing#1 (closed) Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net