configure.ac: check for png_set_rows in libpng because early versions

 don't have it

configure.ac

@@ -2324,11 +2324,13 @@ dnl
 dnl   png
 dnl
 AC_CHECK_HEADERS(png.h, [
+  AC_CHECK_LIB(png, png_set_rows, [
     VLC_ADD_LDFLAGS([logo],[-lpng -lz])
     VLC_ADD_PLUGINS([logo])
     VLC_ADD_LDFLAGS([svcdsub],[-lpng -lz])
     VLC_ADD_LDFLAGS([cvdsub],[-lpng -lz -lm])
     AC_DEFINE(HAVE_LIBPNG, [], [Define if you have the PNG library: libpng])])
+  ])
 
 dnl
 dnl  Video plugins