Improve emscripten download, install and auto-detection

lib/naclports/source_package.py

 # 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.
 
 import contextlib
 import fnmatch
 import os
 import re
 import subprocess
 import sys
@@ skipping 358 matching lines @@
             sys.stdout.write(log_file.read())
         raise
 
     duration = FormatTimeDelta(time.time() - start)
     util.LogHeading('Build complete', ' [took %s]' % duration)
 
   def RunBuildSh(self):
     build_port = os.path.join(paths.TOOLS_DIR, 'build_port.sh')
     cmd = [build_port]
 
+    if self.config.toolchain == 'emscripten':
+      util.SetupEmscripten()
     env = os.environ.copy()
     env['TOOLCHAIN'] = self.config.toolchain
     env['NACL_ARCH'] = self.config.arch
     env['NACL_DEBUG'] = self.config.debug and '1' or '0'
     env['NACL_SDK_ROOT'] = util.GetSDKRoot()
-    if self.config.toolchain == 'emscripten':
-      env['EMSCRIPTEN'] = util.GetEmscriptenRoot()
     rtn = subprocess.call(cmd,
                           stdout=sys.stdout,
                           stderr=sys.stderr,
                           cwd=self.root,
                           env=env)
     if rtn != 0:
       raise Error("Building %s: failed." % (self.NAME))
 
   def Download(self, force_mirror=None):
     """Download upstream sources and verify integrity."""
@@ skipping 390 matching lines @@
   if os.path.isdir(package_name):
     return SourcePackage(package_name, config)
 
   for subdir in DEFAULT_LOCATIONS:
     pkg_root = os.path.join(paths.NACLPORTS_ROOT, subdir, package_name)
     info = os.path.join(pkg_root, 'pkg_info')
     if os.path.exists(info):
       return SourcePackage(pkg_root, config)
 
   raise Error("Package not found: %s" % package_name)