[libgxps] Fix building with libpng15

[Carlos Garcia Campos <carlosgc src gnome org>]

commit 5a63ac53db13fe75b9105ee7b4c03dfeaac54247
Author: Alexandre Rostovtsev <tetromino gentoo org>
Date:   Wed Nov 23 13:24:50 2011 -0500

    Fix building with libpng15
    
    https://bugzilla.gnome.org/show_bug.cgi?id=664666

 tools/gxps-png-writer.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/tools/gxps-png-writer.c b/tools/gxps-png-writer.c
index 7d78ec9..369e0e0 100644
--- a/tools/gxps-png-writer.c
+++ b/tools/gxps-png-writer.c
@@ -23,6 +23,11 @@
 #include <png.h>
 #include <stdint.h>
 
+/* starting with libpng15, png.h no longer #includes zlib.h */
+#ifndef Z_BEST_COMPRESSION
+#define Z_BEST_COMPRESSION 9
+#endif
+
 struct _GXPSPngWriter {
 	GObject parent;