Allow re-targeting of naclports install

libraries/boost_1_47_0/nacl-boost_1_47_0.sh

 #!/bin/bash
 # Copyright (c) 2011 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.
 #
 
 # nacl-boost_1_47_0.sh
 #
 # usage:  nacl-boost_1_47_0.sh
 #
 # this script downloads, patches, and builds boost for Native Client
 #
 
 source pkg_info
 source ../../build_tools/common.sh
 
 CustomConfigureStep() {
   ChangeDir ${NACL_PACKAGES_REPOSITORY}/${PACKAGE_NAME}
   echo "using gcc : 4.4.3 : ${NACLCXX} ;" > tools/build/v2/user-config.jam
 }
 
 CustomBuildStep() {
   ./bootstrap.sh
 
   # TODO(eugenis): build dynamic libraries, too
   if [ $NACL_GLIBC = "1" ] ; then
     ./bjam install \
-      --prefix=${NACL_SDK_USR} \
+      --prefix=${NACLPORTS_PREFIX} \
       link=static \
       -d+2 \
       --without-python \
       --without-signals \
       --without-mpi
   else
     ./bjam install \
-      --prefix=${NACL_SDK_USR} \
+      --prefix=${NACLPORTS_PREFIX} \
       link=static \
       -d+2 \
       --with-date_time \
       --with-program_options
   fi
 }
 
 CustomPackageInstall() {
   DefaultPreInstallStep
   DefaultDownloadBzipStep
   DefaultExtractBzipStep
   DefaultPatchStep
   CustomConfigureStep
   CustomBuildStep
   # Installation is done in the build step.
   DefaultCleanUpStep
 }
 
 
 CustomPackageInstall
 exit 0