Update debian/rules file

Update debian/rules file

In the course of updating the package to the latest Debian
Standards-Version I wanted to bring the debian/rules file up-to-date.
I generated a new template with dh_make(1) with the --addmissing option
and merged the header of template to the existing rules file. (Changing
the rest is work-in-progress and will follow at some later time).

The new template introduces the DPKG_EXPORT_BUILDFLAGS variable which
exports "all buildflags into the environment (where they can be picked
up by your build system)", see: dpkg-buildflags(1).

Exporting the buildflags caused some problems, because of a bug in the
"./configure"-script. This has been fixed with the patch, which has been
introduced in commit 93406119.