Display warning if AC_PROG_INTLTOOL is used (bug #8930)

Similar to AC_PROG_LIBTOOL, which has been replaced with LT_PREREQ
and LT_INIT, AC_PROG_INTLTOOL has been replaced with IT_PROG_INTLTOOL.
This commit makes xdt-autogen recommend intltool >= 0.35.0 and displays
a warning if AC_PROG_INTLTOOL is still being used in a configure script
template.

Original patch by Samuli Suominen <ssuominen@gentoo.org>