build 2 similar binary packages from one source tree

[Osamu Aoki <osamu@debian.org>]

Hi,

maildrop source tree can be build with different build option to produce
maildrop program in 2 ways for each arch.

 * set GID mail without restricted caller (maildrop)
 * set UID root with restricted caller for courier-mta
   (maildrop-courier) -- This is now missing in archive but we used to
   have it.

Of course we can build 2 source packages to do this.  But is there any
easy clean way to do this without making 2 source packages with
identical upstream tarball.

Any pointer to a simple example which uses autotools as its build script
is appreciated.  (Program example simpler than glibc or linux kernel is
appreciated.)

Regards,

Osamu