* compile PNG only when ZLIB is available

Originally committed as revision 1609 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/allformats.c

@@ -75,7 +75,9 @@ void av_register_all(void)
     av_register_image_format(&ppm_image_format);
     av_register_image_format(&pgmyuv_image_format);
     av_register_image_format(&yuv_image_format);
+#ifdef CONFIG_ZLIB
     av_register_image_format(&png_image_format);
+#endif
     av_register_image_format(&jpeg_image_format);
     av_register_image_format(&gif_image_format);