Reduce the number of times we build a host copy of python.

ports/python/build.sh

 # Copyright (c) 2013 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.
 
 EXECUTABLES=python${NACL_EXEEXT}
 
 # This build relies on certain host binaries and python's configure
 # requires us to set --build= as well as --host=.
-HOST_BUILD_DIR=${WORK_DIR}/build-nacl-host
-
-BuildHostPython() {
-  # Seems that python builds to python.exe on mac (!) so they
-  # can avoid a name conflict with the "Python" folder, since
-  # the mac filesystem is case insensistive.
-  if [ ${OS_NAME} != "Linux" ]; then
-    local EXT=.exe
-  else
-    local EXT=
-  fi
-
-  MakeDir ${HOST_BUILD_DIR}
-  ChangeDir ${HOST_BUILD_DIR}
-  if [ -f python${EXT} -a -f Parser/pgen ]; then
-    return
-  fi
-  LogExecute ${SRC_DIR}/configure
-  LogExecute make -j${OS_JOBS} python${EXT} Parser/pgen
-}
+HOST_BUILD_DIR=${WORK_DIR}/build_host
 
 ConfigureStep() {
-  BuildHostPython
-  ChangeDir ${BUILD_DIR}
   # We pre-seed configure with certain results that it cannot determine
   # since we are doing a cross compile.  The $CONFIG_SITE file is sourced
   # by configure early on.
   export CROSS_COMPILE=true
   export CONFIG_SITE=${START_DIR}/config.site
   # Disable ipv6 since configure claims it requires a working getaddrinfo
   # which we do not provide.  TODO(sbc): remove this once nacl_io supports
   # getaddrinfo.
   EXTRA_CONFIGURE_ARGS="--disable-ipv6"
   EXTRA_CONFIGURE_ARGS+=" --with-suffix=${NACL_EXEEXT}"
   EXTRA_CONFIGURE_ARGS+=" --build=x86_64-linux-gnu"
   export LIBS="-ltermcap"
   if [ "${NACL_LIBC}" = "newlib" ]; then
     LIBS+=" -lglibc-compat"
   fi
   DefaultConfigureStep
   if [ "${NACL_LIBC}" = "newlib" ]; then
     # For static linking we copy in a pre-baked Setup.local
     LogExecute cp ${START_DIR}/Setup.local Modules/
   fi
 }
 
 BuildStep() {
   export CROSS_COMPILE=true
-  export MAKEFLAGS="PGEN=${HOST_BUILD_DIR}/Parser/pgen"
+  export MAKEFLAGS="PGEN=${NACL_HOST_PYBUILD}/Parser/pgen"
   SetupCrossEnvironment
   DefaultBuildStep
 }
 
 InstallStep() {
   export CROSS_COMPILE=true
   DefaultInstallStep
 }
 
 TestStep() {
   if [ ${NACL_ARCH} = "pnacl" ]; then
     local pexe=python${NACL_EXEEXT}
     local script=python
     # on Mac/Windows the folder called Python prevents us from creating a
     # script called python (lowercase).
     if [ ${OS_NAME} != "Linux" ]; then
       script+=".sh"
     fi
     TranslateAndWriteSelLdrScript ${pexe} x86-64 python.x86-64.nexe ${script}
   fi
 }