diff --git a/ANNOUNCE b/ANNOUNCE
index 27b386033..8c1f24cce 100644
--- a/ANNOUNCE
+++ b/ANNOUNCE
@@ -1,5 +1,5 @@
 
-Libpng 1.5.0beta57 - December 8, 2010
+Libpng 1.5.0beta57 - December 9, 2010
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -226,7 +226,7 @@ version 1.5.0beta24 [May 7, 2010]
     offset of the png_ptr->rowbuf pointer into png_ptr->big_row_buf.
   Added more blank lines for readability.
 
-version 1.5.0beta25 [December 8, 2010]
+version 1.5.0beta25 [December 9, 2010]
   In pngpread.c: png_push_have_row() add check for new_row > height
   Removed the now-redundant check for out-of-bounds new_row from example.c
 
@@ -459,7 +459,7 @@ Version 1.5.0beta56 [December 7, 2010]
   Fixed some incorrect "=" macro names in pnglibconf.dfa
   Included documentation of changes in 1.5.0 from 1.4.x in libpng-manual.txt
 
-Version 1.5.0beta57 [December 8, 2010]
+Version 1.5.0beta57 [December 9, 2010]
   Documented the pngvalid gamma error summary with additional comments and
     print statements.
   Improved missing symbol handling in checksym.awk; symbols missing in both
@@ -470,6 +470,7 @@ Version 1.5.0beta57 [December 8, 2010]
   Enabled the strip16 tests in pngvalid.`
   Don't display test results (except PASS/FAIL) when running "make test".
     Instead put them in pngtest-log.txt
+  Added "--with-zprefix=<string>" to configure.ac
 
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diff --git a/CHANGES b/CHANGES
index 71e7fd435..4c3a7253d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -3097,7 +3097,7 @@ Version 1.5.0beta56 [December 7, 2010]
   Fixed some incorrect "=" macro names in pnglibconf.dfa
   Included documentation of changes in 1.5.0 from 1.4.x in libpng-manual.txt
 
-Version 1.5.0beta57 [December 8, 2010]
+Version 1.5.0beta57 [December 9, 2010]
   Documented the pngvalid gamma error summary with additional comments and
     print statements.
   Improved missing symbol handling in checksym.awk; symbols missing in both
@@ -3108,6 +3108,7 @@ Version 1.5.0beta57 [December 8, 2010]
   Enabled the strip16 tests in pngvalid.`
   Don't display test results (except PASS/FAIL) when running "make test".
     Instead put them in pngtest-log.txt
+  Added "--with-zprefix=<string>" to configure.ac
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit
diff --git a/configure.ac b/configure.ac
index c9d111946..8fd400f34 100644
--- a/configure.ac
+++ b/configure.ac
@@ -57,7 +57,15 @@ AC_STRUCT_TM
 AC_FUNC_STRTOD
 AC_CHECK_FUNCS([memset], , AC_ERROR([memset not found in libc]))
 AC_CHECK_FUNCS([pow], , AC_CHECK_LIB(m, pow, , AC_ERROR([cannot find pow])) )
-AC_CHECK_LIB(z, zlibVersion, , AC_ERROR([zlib not installed]))
+AC_ARG_WITH(zprefix,
+   AC_HELP_STRING([--with-zprefix],
+                  [prefix that may have been used in installed zlib]),
+                  [zprefix=${withval}],
+                  [zprefix='z_'])
+AC_CHECK_LIB(z, zlibVersion, ,
+    AC_CHECK_LIB(z, ${zprefix}zlibVersion, ,
+                 AC_ERROR([zlib not installed])))
+
 
 LIBPNG_DEFINES=-DPNG_CONFIGURE_LIBPNG
 LIBPNG_DEFINES=$LIBPNG_DEFINES