From John Bowler's post on zlib-devel:
http://mail.madler.net/pipermail/zlib-devel_madler.net/2010-April/002227.html

Confirmed upstream:
http://mail.madler.net/pipermail/zlib-devel_madler.net/2010-July/002310.html
--- zlib-1.2.5.orig/zlib.h	2010-04-20 04:12:48.000000000 +0000
+++ zlib-1.2.5/zlib.h	2010-07-09 22:10:33.392367554 +0000
@@ -1578,7 +1578,7 @@
 #  define gzoffset gzoffset64
 #  define adler32_combine adler32_combine64
 #  define crc32_combine crc32_combine64
-#  ifdef _LARGEFILE64_SOURCE
+#  ifndef _LARGEFILE64_SOURCE
      ZEXTERN gzFile ZEXPORT gzopen64 OF((const char *, const char *));
      ZEXTERN z_off_t ZEXPORT gzseek64 OF((gzFile, z_off_t, int));
      ZEXTERN z_off_t ZEXPORT gztell64 OF((gzFile));