fontconfig: don't "clean up" on new installs

This is a re-reporting of Debian #636173, since the actual failure is only triggered on Ubuntu.

debian/fontconfig.postinst includes this snippet:

  if dpkg --compare-versions "$2" lt 2.4.0-1; then
  printf "Cleaning up old fontconfig caches... "
  for dir in /usr/share/fonts /var/lib/defoma/fontconfig.d /usr/X11R6/lib/X11/fonts /usr/local/share/fonts ; do
        if [ -d $dir ]; then
            find $dir -name fonts.cache-1 -exec rm -f \{\} \;
            find $dir -depth -mindepth 1 -type d -exec rmdir --ignore-fail-on-non-empty \{\} \;
        fi
  done
  ...

This block gets run when fontconfig is initially installed, as well as when upgrading from fontconfig (<< 2.4.0-1). This interacts poorly with Ubuntu's ttf-mscorefonts-installer package, which includes an empty /usr/share/fonts/truetype/msttcorefonts directory and writes files to that directory in the postinst. If you install fontconfig and ttf-mscorefonts-installer in the same apt run, the directory will get unpacked, fontconfig's postinst will rmdir it, and ttf-mscorefont-installer's postinst will get really confused.

I proposed to Debian that it would suffice to just use "lt-nl" instead of "lt", so that an empty $2 (a new install) causes dpkg
--compare-versions to return false. That way the cleanup code only runs on upgrade from the old version of fontconfig, where it is presumably actually needed, and not on first install.

For more context, see
https://bugs.launchpad.net/ubuntu/+source/msttcorefonts/+bug/670629
comments 14 (which prompted packaging the empty directory) and 29
http://debathena.mit.edu/trac/ticket/975
comments "Pretty sure this is entirely different..." and "Anders and I found the bug..."