Add libvpx port

ports/libvpx/build.sh

+# Copyright 2015 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.
+
+OS_JOBS=1
+
+if [[ ${NACL_LIBC} == newlib ]]; then
+  NACLPORTS_CXXFLAGS+=" -std=gnu++11"
+fi
+
+if [[ ${NACL_ARCH} == arm && ${TOOLCHAIN} == glibc ]]; then
+  # Force -O2 rather then -O3 to work around arm gcc bug
+  EXTRA_CONFIGURE_ARGS="--enable-small"
+fi
+
+SetOptFlags() {
+  # libvps sets it own optimisation flags
+  return
+}
+
+ConfigureStep() {
+  SetupCrossEnvironment
+  LogExecute ${SRC_DIR}/configure --target=generic-gnu --cpu=le32 \
+      --disable-unit-tests --prefix=${PREFIX} \
+      --extra-cflags="${NACLPORTS_CPPFLAGS}" ${EXTRA_CONFIGURE_ARGS:-}
+}