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

build_tools/common.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.
 #
 # Environment variable NACL_ARCH should be set to one of the following
 # values: i686 x86_64 pnacl arm
 
 
 # NAMING CONVENTION
 # =================
@@ skipping 97 matching lines @@
 NACL_BUILD_SUBDIR=build
 NACL_INSTALL_SUBDIR=install
 
 # output directories
 readonly NACL_PACKAGES_OUT=${NACL_SRC}/out
 readonly NACL_PACKAGES_ROOT=${NACL_PACKAGES_OUT}/packages
 readonly NACL_PACKAGES_BUILD=${NACL_PACKAGES_OUT}/build
 readonly NACL_PACKAGES_PUBLISH=${NACL_PACKAGES_OUT}/publish
 readonly NACL_PACKAGES_CACHE=${NACL_PACKAGES_OUT}/cache
 readonly NACL_PACKAGES_STAMPDIR=${NACL_PACKAGES_OUT}/stamp
-readonly NACL_HOST_PYROOT=${NACL_PACKAGES_BUILD}/host_python_root
+readonly NACL_HOST_PYROOT=${NACL_PACKAGES_BUILD}/python-host/install_host
+readonly NACL_HOST_PYBUILD=${NACL_PACKAGES_BUILD}/python-host/build_host
 readonly NACL_HOST_PYTHON=${NACL_HOST_PYROOT}/bin/python2.7
 readonly NACL_DEST_PYROOT=${NACL_PREFIX}
 readonly SITE_PACKAGES="lib/python2.7/site-packages/"
 
 # The components of package names cannot contain underscore
 # characters so use x86-64 rather then x86_64 for arch component.
 if [ "${NACL_ARCH}" = "x86_64" ]; then
   PACKAGE_SUFFIX="_x86-64"
 else
   PACKAGE_SUFFIX="_${NACL_ARCH}"
@@ skipping 1554 matching lines @@
 # These functions are called when this script is imported to do
 # any essential checking/setup operations.
 ######################################################################
 CheckToolchain
 CheckPatchVersion
 CheckSDKVersion
 PatchSpecsFile
 InjectSystemHeaders
 InstallConfigSite
 GetRevision