Overhaul debian/rules

Overhaul debian/rules

Remove the normal CFLAGS setting and instead pass --enable-debug to
configure.  dh_strip will remove the debugging symbols if appropriate.

Use the right Autoconf logic to handle cross-builds.

Use stamp files for configure and install and better logic for
optionally running make clean.

Always copy config.guess and config.sub before running configure and
delete them in the clean target.

Remove unnecessary debhelper programs.

Install documentation from the build directory.