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>
Fix out of source building (bug #10899)

Depending on files generated during build via $(srcdir) breaks builds
that take place outside the source directory. This commit fixes one such
mistake in scripts/Makefile.am that refers to $(srcdir)/xdt-autogen.in.