[RFA] Add support for --without-zlib

[Tom Tromey]

>>>>> "Joel" == Joel Brobecker <brobecker@adacore.com> writes:

>> 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.

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

FWIW, GCC has --with-system-zlib.
It would be nice not to wind up with multiple ways to configure this
same thing.

Tom