Fix linking with png support.

  - Rene Rebe <rene@rocklinux.org>

--- gnuplot-4.0.0/configure.orig	2004-05-04 12:54:07.214955360 +0200
+++ gnuplot-4.0.0/configure	2004-05-04 12:54:23.630459824 +0200
@@ -9860,7 +9860,7 @@
 if test "$with_png" != no -a "$have_gd_png" != yes; then
   CPPFLAGS="$CPPFLAGS $libpng_CPPFLAGS"
   LDFLAGS="$LDFLAGS $libpng_LDFLAGS"
-  TERMLIBS="$TERMLIBS -lpng"
+  TERMLIBS="$TERMLIBS -lpng -lz"
 fi