libtheora: don't build examples

Original motivation for this was that they use ftime(3), an obsolete
function that caused the build to fail for NetBSD, but I think we
should just disable them everywhere, because it's unlikely people need
the compiled examples to be installed on their system.

I did a search for the only useful-sounding example, png2theora, on
<https://codesearch.debian.net/>, and the only use I found of it was
blktrace, which can use either png2theora or ffmpeg.  Our own blktrace
package doesn't depend on libtheora anyway.

OpenSUSE and Void Linux both also disable the examples.

pkgs/development/libraries/libtheora/default.nix

@@ -17,6 +17,8 @@ stdenv.mkDerivation rec {
     })
   ];
 
+  configureFlags = [ "--disable-examples" ];
+
   outputs = [ "out" "dev" "devdoc" ];
   outputDoc = "devdoc";