Add support for new pnacl native toolchain driver

ports/python_modules/matplotlib/build.sh

 # Copyright (c) 2014 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}"
 BuildStep() {
   NACLPORTS_CFLAGS+=" -fexceptions"
   DefaultPythonModuleBuildStep
 }
 
 InstallStep() {
   DefaultPythonModuleInstallStep
 }
 
 ConfigureStep() {
   return
 }