configure.ac: Reduce zlib requirement from 1.2.8 to 1.2.3.

configure.ac: Reduce zlib requirement from 1.2.8 to 1.2.3.

Testing with zlib versions 1.2.{3,4,5,6,7,8} showed no difference in
functionality, correctness, or zlib API usage and 1.2.3 is the oldest
version available in still actively deployed production Linux
distributions (RHEL/CentOS 6 and SuSE 11).

Build 17.1.1 against the system supplied zlib-devel packages for 1.2.3
in EL6 and 1.2.7 on EL7. I then swapped out the zlib version at runtime
via LD_LIBRARY_PATH with ones build from the release tarballs from
zlib.net

Testwise - I ran the piglit shader profile with --quick addded to the
tests since I figured that would exercise the shader cache, which would
in turn use zlib.

Signed-off-by: Chuck Atkins <chuck.atkins@kitware.com>
Cc: 17.1 <mesa-stable@lists.freedesktop.org>
Cc: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
[Emil Velikov: add hunk about version/piglit testing]
Acked-by: Emil Velikov <emil.velikov@collabora.com>