autogen.sh: fix xfce4-dev-tools dependency version

configure.ac.in requires newer xfce4-dev-tools, otherwise build breaks in various ways, eg. config.rpath isn't generated (needs extra autoreconf run) and makefile variables not filled correctly:

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

Closes: #234 (closed) Closes: #235 (closed) Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net