Some time ago the mozilla maintainers found it necessary to #ifndef
out a line of zutil.h when building for VMS:
#ifdef HAVE_STRERROR
+#ifndef VMS
extern char *strerror OF((int));
+#endif
# define zstrerror(errnum) strerror(errnum)
Please make this change to the zlib source.
See mozilla's bugzilla bug #226733.
Glenn