gtk+ configure.in patch

[Sander Vesik <Sander Vesik Sun COM>]

The following patch makes gtk+ configure correctly fail with an error if
libpng cannot be found instead of build later breaking.

Ok to commit?

	Sander

	I see a dark sail on the horizon
	Set under a dark cloud that hides the sun
	Bring me my Broadsword and clear understanding

	----------% cut here %------------

Index: configure.in
===================================================================
RCS file: /cvs/gnome/gtk+/configure.in,v
retrieving revision 1.244
diff -u -r1.244 configure.in
--- configure.in	2002/01/14 01:39:24	1.244
+++ configure.in	2002/01/15 20:07:17
@@ -642,7 +642,7 @@
       [AC_CHECK_HEADER(png.h,
         png_ok=yes,
         png_ok=no)],
-      AC_MSG_WARN(*** PNG loader will not be built (PNG library not found) ***), -lz -lm)
+      AC_MSG_ERROR(*** PNG loader can not be built (PNG library not found) ***), -lz -lm)
     if test "$png_ok" = yes; then
       AC_MSG_CHECKING([for png_structp in png.h])
       AC_TRY_COMPILE([#include <png.h>],