Correctly probe for -lpng and supply -L$libdir into LDFLAGS when needed.

svn path=/head/; revision=7738

math/gnuplot/files/patch-aa

@@ -1,6 +1,6 @@
---- configure.orig	Sun Jan 12 13:16:51 1997
+--- configure	Mon May 26 18:28:46 1997
-+++ configure	Sun Jan 12 13:24:04 1997
++++ configure	Sun Aug 31 23:26:12 1997
-@@ -922,6 +922,7 @@
+@@ -1029,6 +1029,7 @@
    LASERGNU=lasergnu_noinstall
  fi
  
@@ -8,3 +8,20 @@
  
  maybe_linuxdriver=
  # Check whether --with-linux-vga or --without-linux-vga was given.
+@@ -3419,7 +3420,7 @@
+   echo $ac_n "(cached) $ac_c" 1>&6
+ else
+   ac_save_LIBS="$LIBS"
+-LIBS="-lpng -lz $LIBS"
++LIBS="-L$libdir -lpng -lz $LIBS"
+ cat > conftest.$ac_ext <<EOF
+ #line 3425 "configure"
+ #include "confdefs.h"
+@@ -3450,6 +3451,7 @@
+ #define HAVE_LIBPNG 1
+ EOF
+  TERMLIBS="$TERMLIBS -lpng -lz"
++ LDFLAGS="$LDFLAGS -L$libdir"
+     ac_save_CPPFLAGS="$CPPFLAGS"; CPPFLAGS="$CPPFLAGS -I$includedir"
+     for ac_hdr in png.h zlib.h
+ do