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

ports/ruby/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.
 
 MAKE_TARGETS="pprogram"
 INSTALL_TARGETS="install-nodoc"
 
 EXECUTABLES="ruby${NACL_EXEEXT} pepper-ruby${NACL_EXEEXT}"
 
 ConfigureStep() {
   # We need to build a host version of ruby for use during the nacl
   # build.
-  HOST_BUILD=${WORK_DIR}/build-nacl-host
+  HOST_BUILD=${WORK_DIR}/build_host
   if [ ! -x ${HOST_BUILD}/inst/bin/ruby ]; then
     Banner "Building ruby for host"
     MakeDir ${HOST_BUILD}
     ChangeDir ${HOST_BUILD}
     CFLAGS="" LDFLAGS="" LogExecute ${SRC_DIR}/configure --prefix=$PWD/inst
     LogExecute make -j${OS_JOBS} miniruby
     LogExecute make -j${OS_JOBS} install-nodoc
     cd -
   fi
 
@@ skipping 14 matching lines @@
     # does not know about "pnacl".  It only knows about "le32-nacl".
     # Unfortunately, most of the config.subs here are so old that
     # it doesn't know about that "le32" either.  So we just say "nacl".
     conf_host="nacl"
   fi
 
   SetupCrossEnvironment
   LogExecute ${SRC_DIR}/configure \
     --host=${conf_host} \
     --prefix=/ \
-    --with-baseruby=${WORK_DIR}/build-nacl-host/inst/bin/ruby \
+    --with-baseruby=${HOST_BUILD}/inst/bin/ruby \
     --with-http=no \
     --with-html=no \
     --with-ftp=no \
     --${NACL_OPTION}-mmx \
     --${NACL_OPTION}-sse \
     --${NACL_OPTION}-sse2 \
     --${NACL_OPTION}-asm \
     --with-x=no \
     ${EXTRA_CONFIGURE_ARGS}
 }
 
 BuildStep() {
   DefaultBuildStep
   if [ $NACL_ARCH == "pnacl" ]; then
     # Just write the x86-64 version out for now.
     TranslateAndWriteSelLdrScript ruby.pexe x86-64 ruby.x86-64.nexe ruby
   fi
 }
 
 InstallStep() {
   DESTDIR=${DESTDIR}/${PREFIX}
   DefaultInstallStep
 }