[RFA] Add support for --without-zlib (was --disable-zlib)

[Joel Brobecker]

> It should be more systematic to use AC_ARG_WITH than AC_ARG_ENABLE as it is
> about a dependency on external software as described in the autoconf manual.

Makes sense, indeed.  I wanted to avoid --with-zlib because I wanted
to avoid the confusion with a possible --with-zlib-prefix.

> And I would prefer to be also able to force the requirement - it makes the
> vendor packaging sanity checking of prerequisited packages easier.

Agreed.

I will post a new set of patches later.

-- 
Joel