LibPNG 1.2 is too old, so link against libpng instead

Author: Michael Homer <michael@gobolinux.org>
--- devil-1.7.8/m4/devil-definitions.m4.orig	2012-03-10 10:03:46.013973453 +1300
+++ devil-1.7.8/m4/devil-definitions.m4	2012-03-10 10:03:57.813974520 +1300
@@ -244,12 +244,12 @@
 
 AC_DEFUN([SETTLE_PNG],
          [DEVIL_IL_LIB([png.h],
-                       [png12]) 
-          AS_IF([test "x$have_png12" = "xno"],
+                       [png]) 
+          AS_IF([test "x$have_png" = "xno"],
                 [DEVIL_IL_LIB([png.h],
                               [png]) 
                  lib_test_result="$have_png"],
-                [lib_test_result="$have_png12"]) 
+                [lib_test_result="$have_png"]) 
           AS_IF([test "x$lib_test_result" = "xyes"],
                 [MAYBE_OPTIONAL_DEPENDENCY([IL],
                                            [libpng]) ]) ])