allow building non-system cairo with system libpng14

cairo/cairo-1.8.0.patch

@@ -186,6 +186,15 @@ diff -ur misc/build/cairo-1.8.0.org/configure misc/build/cairo-1.8.0/configure
  
  
  { echo "$as_me:$LINENO: checking for X" >&5
+@@ -31723,7 +31723,7 @@
+ 
+   if test "x$png_REQUIRES" = x; then
+     # libpng13 is GnuWin32's libpng-1.2.8 :-(
+-    for l in libpng12 libpng13 libpng10 ; do
++    for l in libpng12 libpng13 libpng14 libpng10 ; do
+       if $PKG_CONFIG --exists $l ; then
+         png_REQUIRES=$l
+         use_png=yes
 @@ -34097,7 +34129,7 @@
  
      # The ps backend requires zlib.