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

ports/python/build.sh

Index: ports/python/build.sh
diff --git a/ports/python/build.sh b/ports/python/build.sh
index bc3f1435f480c657f2c6b97cd300d3d423380031..3ae7bb1987d1752396b3e151df80b99c8db42e9c 100644
--- a/ports/python/build.sh
+++ b/ports/python/build.sh
@@ -6,30 +6,9 @@ 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.
@@ -54,7 +33,7 @@ ConfigureStep() {
 
 BuildStep() {
   export CROSS_COMPILE=true
-  export MAKEFLAGS="PGEN=${HOST_BUILD_DIR}/Parser/pgen"
+  export MAKEFLAGS="PGEN=${NACL_HOST_PYBUILD}/Parser/pgen"
   SetupCrossEnvironment
   DefaultBuildStep
 }