Add top level build script for building packages.

ports/mesagl/build.sh

 #!/bin/bash
 # Copyright (c) 2013 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.
 
-source pkg_info
-source ../../build_tools/common.sh
+BUILD_DIR=${START_DIR}
 
 BuildStep() {
   # The sample is built using the NaCl SDK common.mk system.
   # We override $(OUTBASE) to force the build system to put
   # all its artifacts in ${NACL_PACKAGES_REPOSITORY} rather
   # than alongside the Makefile.
   export OUTBASE=${NACL_PACKAGES_REPOSITORY}/${PACKAGE_DIR}
   export NACLPORTS_INCLUDE
   export NACL_PACKAGES_PUBLISH
   if [ "${NACL_ARCH}" = "pnacl" ]; then
     MAKEFLAGS+=" TOOLCHAIN=pnacl"
   elif [ "${NACL_GLIBC}" = "1" ]; then
     MAKEFLAGS+=" TOOLCHAIN=glibc"
     MAKEFLAGS+=" NACL_ARCH=${NACL_ARCH_ALT}"
   else
     MAKEFLAGS+=" TOOLCHAIN=newlib"
     MAKEFLAGS+=" NACL_ARCH=${NACL_ARCH_ALT}"
   fi
   export MAKEFLAGS
-  ChangeDir ${START_DIR}
   DefaultBuildStep
 }
-
-PackageInstall
-exit 0