--- a/build/moz.configure/init.configure
+++ b/build/moz.configure/init.configure
@@ -249,98 +249,44 @@ def early_options():
             option.env
             for option in __sandbox__._options.itervalues()
             if option.env
         )
     return early_options
 
 early_options = early_options()
 
-# At the moment, moz.configure doesn't have complete knowledge of all the
-# supported options in mozconfig because of all that is still in old.configure.
-# But we don't know all the options that moz.configure knows about until all
-# moz.configure files are executed, so we keep a manual list here, that is
-# checked in old.configure (we'll assume it's the last moz.configure file
-# processed). This is tedious but necessary for now.
-@depends('--help')
-def wanted_mozconfig_variables(help):
-     return set([
-         'AUTOCONF',
-         'AWK',
-         'CC',
-         'CCACHE',
-         'CXX',
-         'COMPILER_WRAPPER',
-         'DISABLE_EXPORT_JS',
-         'DISABLE_SHARED_JS',
-         'DOXYGEN',
-         'DSYMUTIL',
-         'EXTERNAL_SOURCE_DIR',
-         'GENISOIMAGE',
-         'GRADLE',
-         'GRADLE_FLAGS',
-         'GRADLE_MAVEN_REPOSITORY',
-         'HOST_CC',
-         'HOST_CXX',
-         'JS_STANDALONE',
-         'L10NBASEDIR',
-         'MOZILLABUILD',
-         'MOZ_ARTIFACT_BUILDS',
-         'MOZ_BUILD_APP',
-         'MOZ_BUILD_MOBILE_ANDROID_WITH_GRADLE',
-         'MOZ_CALLGRIND',
-         'MOZ_DMD',
-         'MOZ_FMP4',
-         'MOZ_INSTRUMENT_EVENT_LOOP',
-         'MOZ_INSTRUMENTS',
-         'MOZ_JPROF',
-         'MOZ_PROFILING',
-         'MOZ_USE_SYSTRACE',
-         'MOZ_VTUNE',
-         'MOZTTDIR',
-         'PERL',
-         'RPMBUILD',
-         'TAR',
-         'TOOLCHAIN_PREFIX',
-         'UNZIP',
-         'USE_FC_FREETYPE',
-         'WITHOUT_X',
-         'XARGS',
-         'YASM',
-         'ZIP',
-     ])
-
-
-@depends(mozconfig, wanted_mozconfig_variables, '--help')
+@depends(mozconfig, '--help')
 # This gives access to the sandbox. Don't copy this blindly.
 @imports('__sandbox__')
-def mozconfig_options(mozconfig, wanted_mozconfig_variables, help):
+def mozconfig_options(mozconfig, help):
     if mozconfig['path']:
         helper = __sandbox__._helper
         log.info('Adding configure options from %s' % mozconfig['path'])
         for arg in mozconfig['configure_args']:
             log.info('  %s' % arg)
             # We could be using imply_option() here, but it has other
             # contraints that don't really apply to the command-line
             # emulation that mozconfig provides.
             helper.add(arg, origin='mozconfig', args=helper._args)
 
         def add(key, value):
-            # See comment above wanted_mozconfig_variables
-            if key in wanted_mozconfig_variables:
+            if key.isupper():
                 arg = '%s=%s' % (key, value)
                 log.info('  %s' % arg)
                 helper.add(arg, origin='mozconfig', args=helper._args)
 
         for key, value in mozconfig['env']['added'].iteritems():
             add(key, value)
         for key, (_, value) in mozconfig['env']['modified'].iteritems():
             add(key, value)
         for key, value in mozconfig['vars']['added'].iteritems():
-            add(key, value)
+            # mozconfig_loader adds _IS_SET variables that are irrelevant
+            if not key.endswith('_IS_SET'):
+                add(key, value)
         for key, (_, value) in mozconfig['vars']['modified'].iteritems():
             add(key, value)
 
 
 # Mozilla-Build
 # ==============================================================
 option(env='MOZILLABUILD', nargs=1,
        help='Path to Mozilla Build (Windows-only)')

--- a/build/moz.configure/old.configure
+++ b/build/moz.configure/old.configure
@@ -53,36 +53,16 @@ def autoconf(mozconfig, autoconf):
     if not os.path.exists(autoconf):
         die('Could not find autoconf 2.13 at %s', autoconf)
 
     return autoconf
 
 set_config('AUTOCONF', autoconf)
 
 
-# See comment in mozconfig_options() from build/moz.configure/init.configure
-@template
-# This gives access to the sandbox. Don't copy this blindly.
-@imports('__sandbox__')
-def check_mozconfig_variables():
-    # This escapes the sandbox. Don't copy this. This is only here because it
-    # is a one off until old-configure is gone.
-    all_options = __sandbox__._options.itervalues()
-
-    @depends(early_options, wanted_mozconfig_variables)
-    def check_mozconfig_variables(early_options, wanted_mozconfig_variables):
-        for option in all_options:
-            if (option.env and option.env not in early_options and
-                    option.env not in wanted_mozconfig_variables):
-                die('You need to add `%s` to the `wanted_mozconfig_variables` '
-                    'list in build/moz.configure/init.configure.', option.env)
-
-check_mozconfig_variables()
-
-
 @depends('OLD_CONFIGURE', mozconfig, autoconf, check_build_environment, shell,
          old_configure_assignments, build_project)
 @imports(_from='__builtin__', _import='open')
 @imports(_from='__builtin__', _import='print')
 @imports('glob')
 @imports('itertools')
 @imports('subprocess')
 # Import getmtime without overwriting the sandbox os.path.
@@ -462,8 +442,29 @@ def post_old_configure(raw_config):
         set_old_configure_config(
             k[1:-1], v[1:-1] if isinstance(v, types.StringTypes) else v)
 
     for k, v in dict(raw_config['defines']).iteritems():
         set_old_configure_define(k[1:-1], v[1:-1])
 
     set_old_configure_config('non_global_defines',
                              raw_config['non_global_defines'])
+
+
+# Assuming no other option is declared after this function, handle the
+# env options that were injected by mozconfig_options by creating dummy
+# Option instances and having the sandbox's CommandLineHelper handle
+# them. We only do so for options that haven't been declared so far,
+# which should be a proxy for the options that old-configure handles
+# and that we don't know anything about.
+@depends('--help')
+@imports('__sandbox__')
+@imports(_from='mozbuild.configure.options', _import='Option')
+def remaining_mozconfig_options(_):
+    helper = __sandbox__._helper
+    for arg in helper:
+        if helper._origins[arg] == 'mozconfig':
+            name = arg.split('=', 1)[0]
+            if name.isupper() and name not in __sandbox__._options:
+                option = Option(env=name, nargs='*', help=name)
+                helper.handle(option)
+
+# Please do not add anything after remaining_mozconfig_options()

--- a/moz.configure
+++ b/moz.configure
@@ -161,8 +161,9 @@ check_prog('DSYMUTIL', delayed_getattr(e
 check_prog('GENISOIMAGE', delayed_getattr(extra_programs, 'GENISOIMAGE'),
            allow_missing=True)
 check_prog('RPMBUILD', delayed_getattr(extra_programs, 'RPMBUILD'),
            allow_missing=True)
 
 
 # Fallthrough to autoconf-based configure
 include('build/moz.configure/old.configure')
+# Please do not add anything after the include of old.configure.