Allow re-targeting of naclports install

libraries/zlib-1.2.3/nacl-zlib-1.2.3.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-zlib-1.2.3.sh
 #
 # usage:  nacl-zlib-1.2.3.sh
 #
 # this script downloads, patches, and builds zlib for Native Client 
 #
 
 source pkg_info
 source ../../build_tools/common.sh
 
 
 CustomConfigureStep() {
   Banner "Configuring ${PACKAGE_NAME}"
   ChangeDir ${NACL_PACKAGES_REPOSITORY}/${PACKAGE_NAME}
   # TODO: side-by-side install
   CC=${NACLCC} AR="${NACLAR} -r" RANLIB=${NACLRANLIB} CFLAGS="-Dunlink=puts" ./configure\
-     --prefix=${NACL_SDK_USR}
+     --prefix=${NACLPORTS_PREFIX}
 }
 
 
 CustomPackageInstall() {
   DefaultPreInstallStep
   DefaultDownloadStep
   DefaultExtractStep
   # zlib doesn't need patching, so no patch step
   CustomConfigureStep
   DefaultBuildStep
   DefaultInstallStep
   DefaultCleanUpStep
 }
 
 
 CustomPackageInstall
 exit 0