Add libvpx port

ports/ffmpeg/build.sh

 # Copyright (c) 2012 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 EXECUTABLES="ffmpeg ffmpeg_g ffprobe ffprobe_g"
 
 # needed for RLIMIT_CPU
 EnableGlibcCompat
 
 ConfigureStep() {
@@ skipping 23 matching lines @@
     --enable-cross-compile \
     --disable-inline-asm \
     --disable-ssse3 \
     --disable-mmx \
     --disable-amd3dnow \
     --disable-amd3dnowext \
     --disable-indevs \
     --enable-libmp3lame \
     --enable-libvorbis \
     --enable-libtheora \
+    --enable-libvpx \
     --disable-ffplay \
     --disable-ffserver \
     --disable-demuxer=rtsp \
     --disable-demuxer=image2 \
     --prefix=${PREFIX} \
     ${extra_args}
 }
 
 TestStep() {
   SetupCrossPaths
   LogExecute make testprogs
 }