Allow re-targeting of naclports install

examples/needs_porting/xaos/xaos_tool.sh

 #!/bin/bash
 # Copyright (c) 2008 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that be
 # found in the LICENSE file.
 #
 
 #@ xaos_tool.sh
 #@
 #@ usage:  xaos_tool.sh <mode>
 #@
@@ skipping 99 matching lines @@
 
 # params (tmp, install)
 BuildXaos() {
   Banner "configuring xaos"
   cd $1
   export PATH="$2/bin:${PATH}"
   export CC=${NACLCC}
   export AR=${NACLAR}
   export RANLIB=${NACLRANLIB}
   export LDFLAGS="-static"
-  export CFLAGS=""
+  export CFLAGS="${NACLPORTS_CFLAGS}"
   export CFLAGS="${CFLAGS} -DNACL_DIM_H=${NACL_DIM_H}"
   export CFLAGS="${CFLAGS} -DNACL_DIM_W=${NACL_DIM_W}"
   export CFLAGS="${CFLAGS} -I${DIR_AV_INCLUDE}"
   export LIBS="-L${DIR_AV_LIB} -lav -lsrpc -lpthread"
   rm -rf xaos-build
   # sadly xaos seem wants to be built in-place
   cp -r  XaoS-3.4 xaos-build
   cd xaos-build
   ../XaoS-3.4/configure\
       --with-png=no\
@@ skipping 72 matching lines @@
 
 
 ######################################################################
 # Mode is not handled
 ######################################################################
 
 echo "unknown mode: ${MODE}"
 exit -1