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

ports/gforth/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.
 
 # NOTE: This build doesn't work in parallel.
 OS_JOBS=1
 
 # This build relies on certain host binaries.
-HOST_BUILD_DIR=${WORK_DIR}/build-nacl-host
-HOST_INSTALL_DIR=${WORK_DIR}/install-nacl-host
+HOST_BUILD_DIR=${WORK_DIR}/build_host
+HOST_INSTALL_DIR=${WORK_DIR}/install_host
 
 EXECUTABLES="gforth-ditc${NACL_EXEEXT}"
 MAKE_TARGETS="${EXECUTABLES}"
 
 BuildHostGforth() {
   MakeDir ${HOST_BUILD_DIR}
   ChangeDir ${HOST_BUILD_DIR}
   CC="gcc -m32" \
       LogExecute ${SRC_DIR}/configure --prefix=${HOST_INSTALL_DIR}
   LogExecute make -j${OS_JOBS}
@@ skipping 51 matching lines @@
   LogExecute python ${TOOLS_DIR}/create_term.py gforth.nmf
 
   InstallNaClTerm ${PUBLISH_DIR}
 
   cp ${START_DIR}/gforth.js ${PUBLISH_DIR}/
 
   ChangeDir ${PUBLISH_DIR}
   Remove gforth.zip
   LogExecute zip -r gforth.zip .
 }