Fix FTBFS in qvfb with libpng >= 1.5

(cherry picked from commit af5bc055c2)

tools/qvfb/qanimationwriter.cpp

@@ -101,7 +101,15 @@ public:
 
     void writePNG(const QImage& image)
     {
+#if PNG_LIBPNG_VER_MAJOR>1 || ( PNG_LIBPNG_VER_MAJOR==1 && PNG_LIBPNG_VER_MINOR>=5 )
+#warning XXXtnn not too sure about this
+/* 
+according to png.h, channels is only used on read, not writes, so we
+should be able to comment this out.
+*/
+#else /* LIBPNG 1.5 */
 	info_ptr->channels = 4;
+#endif
 	png_set_sig_bytes(png_ptr, 8); // Pretend we already wrote the sig
 	png_set_IHDR(png_ptr, info_ptr, image.width(), image.height(),
 	    8, image.hasAlphaBuffer()