Add support for new pnacl native toolchain driver

ports/dosbox/build.sh

 # Copyright (c) 2012 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.
 
+NACLPORTS_CPPFLAGS+=" ${NACL_EXCEPTIONS_FLAG}"
 EXECUTABLES=src/dosbox${NACL_EXEEXT}
 MAKE_TARGETS="AR=${NACLAR}"
 
 ConfigureStep() {
   SetupCrossEnvironment
 
   CONFIG_FLAGS="--host=${CONF_HOST} --build=${CONF_BUILD} \
       --prefix=${PREFIX} \
       --with-sdl-prefix=${NACL_TOOLCHAIN_ROOT} \
       --disable-shared \
       --with-sdl-exec-prefix=${NACL_TOOLCHAIN_ROOT}"
 
   LogExecute ${SRC_DIR}/configure ${CONFIG_FLAGS}
 }
 
 InstallStep(){
   MakeDir ${PUBLISH_DIR}
   LogExecute install ${START_DIR}/dosbox.html ${PUBLISH_DIR}
   LogExecute install src/dosbox${NACL_EXEEXT} \
     ${PUBLISH_DIR}/dosbox_${NACL_ARCH}${NACL_EXEEXT}
   local CREATE_NMF="${NACL_SDK_ROOT}/tools/create_nmf.py"
   LogExecute ${CREATE_NMF} -s ${PUBLISH_DIR} ${PUBLISH_DIR}/dosbox_*${NACL_EXEEXT} -o ${PUBLISH_DIR}/dosbox.nmf
 
   if [ "${NACL_ARCH}" = "pnacl" ]; then
     sed -i.bak 's/x-nacl/x-pnacl/' ${PUBLISH_DIR}/dosbox.html
   fi
 }