Detect whether zlib is new enough.  Based on patch from Oliver Jowett.

CONTRIBUTORS.txt

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index d2c3d18..4aa1eea 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -74,3 +74,5 @@
     * Added Swap(), SwapElements(), and RemoveLast() to Reflection interface.
   Alexander Melnikov <alm@sibmail.ru>
     * HPUX support.
+  Oliver Jowett <oliver.jowett@gmail.com>
+    * Detect whether zlib is new enough in configure script.