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 <[email protected]>
Cc: 17.1 <[email protected]>
Cc: Timothy Arceri <[email protected]>
Reviewed-by: Timothy Arceri <[email protected]>
[Emil Velikov: add hunk about version/piglit testing]
Acked-by: Emil Velikov <[email protected]>

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