Fix HAVE_LIBPNG in configure.ac

configure.ac

@@ -3770,7 +3770,7 @@ AC_CHECK_HEADERS(png.h, [
     VLC_ADD_LIBS([png],[-lpng -lz])
     VLC_ADD_PLUGINS([png])
     VLC_ADD_PLUGINS([osdmenu osd_parser])
-    AC_DEFINE(HAVE_LIBPNG, [], [Define if you have the PNG library: libpng])],
+    AC_DEFINE(HAVE_LIBPNG, 1, [Define if you have the PNG library: libpng])],
     [],[-lz])
   LDFLAGS="${LDFLAGS_save}"
   ])