--- a/build/moz.configure/init.configure
+++ b/build/moz.configure/init.configure
@@ -83,16 +83,32 @@ def mozconfig(current_project, mozconfig
     current_project = current_project[0] if current_project else None
     mozconfig = mozconfig[0] if mozconfig else None
     mozconfig = loader.find_mozconfig(env={'MOZCONFIG': mozconfig})
     mozconfig = loader.read_mozconfig(mozconfig, moz_build_app=current_project)
 
     return mozconfig
 
 
+# Hacks related to old-configure
+# ==============================
+
+@depends('--help')
+def old_configure_assignments(help):
+    return []
+
+@template
+def add_old_configure_assignment(var, value):
+    @depends(old_configure_assignments)
+    @advanced
+    def add_assignment(assignments):
+        from mozbuild.shellutil import quote
+        assignments.append('%s=%s' % (var, quote(value)))
+
+
 option(env='PYTHON', nargs=1, help='Python interpreter')
 
 # Setup python virtualenv
 # ==============================================================
 @depends('PYTHON', check_build_environment, mozconfig)
 @advanced
 def virtualenv_python(env_python, build_env, mozconfig):
     import os
@@ -156,16 +172,17 @@ def virtualenv_python(env_python, build_
         sys.exit(subprocess.call([python] + sys.argv))
 
     # We are now in the virtualenv
     import distutils.sysconfig
     if not distutils.sysconfig.get_python_lib():
         error('Could not determine python site packages directory')
 
     set_config('PYTHON', python)
+    add_old_configure_assignment('PYTHON', python)
     return python
 
 
 # Inject mozconfig options
 # ==============================================================
 @template
 @advanced
 def command_line_helper():
@@ -225,27 +242,31 @@ option('--with-external-source-dir', env
          check_build_environment, '--help')
 def include_project_configure(project, external_source_dir, build_env, help):
     if not project:
         error('--enable-project is required.')
 
     base_dir = build_env['TOPSRCDIR']
     if external_source_dir:
         set_config('EXTERNAL_SOURCE_DIR', external_source_dir[0])
+        add_old_configure_assignment('EXTERNAL_SOURCE_DIR',
+                                     external_source_dir[0])
         base_dir = os.path.join(base_dir, external_source_dir[0])
 
     path = os.path.join(base_dir, project[0], 'moz.configure')
     if not os.path.exists(path):
         error('Cannot find project %s' % project[0])
     return path
 
 @depends(include_project_configure, check_build_environment, '--help')
 def build_project(include_project_configure, build_env, help):
-    return os.path.dirname(os.path.relpath(include_project_configure,
-                                           build_env['TOPSRCDIR']))
+    ret = os.path.dirname(os.path.relpath(include_project_configure,
+                                          build_env['TOPSRCDIR']))
+    add_old_configure_assignment('MOZ_BUILD_APP', ret)
+    return ret
 
 
 # This is temporary until js/src/configure and configure are merged.
 @depends(build_project)
 def extra_old_configure_args(build_project):
     if build_project != 'js':
         return []
     return False

--- a/build/moz.configure/old.configure
+++ b/build/moz.configure/old.configure
@@ -57,22 +57,22 @@ def autoconf(mozconfig, autoconf):
     return autoconf
 
 
 option(env='OLD_CONFIGURE', nargs=1, help='Path to the old configure script')
 
 @depends('OLD_CONFIGURE', mozconfig, autoconf, check_build_environment, shell,
          virtualenv_python, compile_environment, build_project,
          extra_old_configure_args, '--with-external-source-dir',
-         '--enable-shared-js')
+         '--enable-shared-js', old_configure_assignments)
 @advanced
 def prepare_configure(old_configure, mozconfig, autoconf, build_env, shell,
                       python, compile_env, build_project,
                       extra_old_configure_args, external_source_dir,
-                      shared_js):
+                      shared_js, old_configure_assignments):
     import glob
     import itertools
     import subprocess
     import sys
     # Import getmtime without overwriting the sandbox os.path.
     from os.path import getmtime
 
     from mozbuild.shellutil import quote
@@ -122,24 +122,18 @@ def prepare_configure(old_configure, moz
             for key, value in mozconfig['vars']['added'].items():
                 print("%s=%s" % (key, quote(value)), file=out)
             for key, (old, value) in mozconfig['vars']['modified'].items():
                 print("%s=%s" % (key, quote(value)), file=out)
             for t in ('env', 'vars'):
                 for key in mozconfig[t]['removed'].keys():
                     print("unset %s" % key, file=out)
 
-        print('PYTHON=%s' % quote(python), file=out)
-        if compile_env:
-            print('COMPILE_ENVIRONMENT=1', file=out)
-        print('MOZ_BUILD_APP=%s' % build_project, file=out)
-        if external_source_dir:
-            print(external_source_dir.format('EXTERNAL_SOURCE_DIR'), file=out)
-        if shared_js:
-            print('JS_SHARED_LIBRARY=1', file=out)
+        for assignment in old_configure_assignments:
+            print(assignment, file=out)
 
     if extra_old_configure_args:
         cmd += extra_old_configure_args
 
     return cmd
 
 
 @template

--- a/js/moz.configure
+++ b/js/moz.configure
@@ -33,16 +33,17 @@ js_option('--disable-export-js', default
           help='Do not mark JS symbols as DLL exported/visible')
 
 @depends('--disable-shared-js', '--disable-export-js')
 def static_js(shared_js, export_js):
     if shared_js:
         if not export_js:
             error('Must export JS symbols when building a shared library.')
         set_config('JS_SHARED_LIBRARY', '1')
+        add_old_configure_assignment('JS_SHARED_LIBRARY', '1')
     else:
         if export_js:
             set_define('STATIC_EXPORTABLE_JS_API', '1')
         else:
             set_define('STATIC_JS_API', '1')
         set_define('MOZ_STATIC_JS', '1')
 
 

--- a/moz.configure
+++ b/moz.configure
@@ -27,16 +27,17 @@ def artifact_builds(value):
 
 option('--disable-compile-environment',
        help='Disable compiler/library checks')
 
 @depends('--disable-compile-environment')
 def compile_environment(value):
     if value:
         set_config('COMPILE_ENVIRONMENT', '1')
+        add_old_configure_assignment('COMPILE_ENVIRONMENT', '1')
     return bool(value)
 
 
 @depends('--help')
 @advanced
 def build_backends_choices(help):
     from mozbuild.backend import backends
     return tuple(backends)