diff --git a/build/macosx/universal/flight.mk b/build/macosx/universal/flight.mk
index 675d9d36bed2..544382da7135 100644
--- a/build/macosx/universal/flight.mk
+++ b/build/macosx/universal/flight.mk
@@ -16,6 +16,7 @@ OBJDIR = $(OBJDIR_ARCH_1)
 endif
 
 topsrcdir = $(TOPSRCDIR)
+DEPTH = $(OBJDIR)
 include $(OBJDIR)/config/autoconf.mk
 
 core_abspath = $(if $(filter /%,$(1)),$(1),$(CURDIR)/$(1))
diff --git a/config/autoconf.mk.in b/config/autoconf.mk.in
index 3a1f0d8f49e1..41f7854eeef7 100644
--- a/config/autoconf.mk.in
+++ b/config/autoconf.mk.in
@@ -1,2 +1,3 @@
+include $(DEPTH)/config/emptyvars.mk
 @ALLSUBSTS@
 include $(topsrcdir)/config/baseconfig.mk
diff --git a/config/emptyvars.mk.in b/config/emptyvars.mk.in
new file mode 100644
index 000000000000..388cf2a35986
--- /dev/null
+++ b/config/emptyvars.mk.in
@@ -0,0 +1 @@
+@ALLEMPTYSUBSTS@
diff --git a/config/moz.build b/config/moz.build
index 8c98eb6c345c..422455a99600 100644
--- a/config/moz.build
+++ b/config/moz.build
@@ -6,6 +6,7 @@
 
 CONFIGURE_SUBST_FILES += [
     'autoconf.mk',
+    'emptyvars.mk',
     'doxygen.cfg',
     'makefiles/test/Makefile',
     'tests/makefiles/autodeps/Makefile',
diff --git a/js/src/config/autoconf.mk.in b/js/src/config/autoconf.mk.in
index 3a1f0d8f49e1..41f7854eeef7 100644
--- a/js/src/config/autoconf.mk.in
+++ b/js/src/config/autoconf.mk.in
@@ -1,2 +1,3 @@
+include $(DEPTH)/config/emptyvars.mk
 @ALLSUBSTS@
 include $(topsrcdir)/config/baseconfig.mk
diff --git a/js/src/config/emptyvars.mk.in b/js/src/config/emptyvars.mk.in
new file mode 100644
index 000000000000..388cf2a35986
--- /dev/null
+++ b/js/src/config/emptyvars.mk.in
@@ -0,0 +1 @@
+@ALLEMPTYSUBSTS@
diff --git a/js/src/configure.in b/js/src/configure.in
index 1e6636dc0fe5..3ec29c04489a 100644
--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -4384,7 +4384,7 @@ AC_SUBST(ac_configure_args)
 dnl Spit out some output
 dnl ========================================================
 
-AC_OUTPUT([js-confdefs.h Makefile config/autoconf.mk])
+AC_OUTPUT([js-confdefs.h Makefile config/autoconf.mk config/emptyvars.mk])
 
 # Produce the js-config script at configure time; see the comments for
 # 'js-config' in Makefile.in.
diff --git a/python/mozbuild/mozbuild/backend/configenvironment.py b/python/mozbuild/mozbuild/backend/configenvironment.py
index 89d8a5684296..dc63edcf4ea5 100644
--- a/python/mozbuild/mozbuild/backend/configenvironment.py
+++ b/python/mozbuild/mozbuild/backend/configenvironment.py
@@ -86,13 +86,15 @@ class ConfigEnvironment(object):
       - ALLDEFINES contains the defines in the form #define NAME VALUE, in
         sorted order, for use in config files, for an automatic listing of
         defines.
-    and another additional subst variable from all the other substs:
+    and two other additional subst variables from all the other substs:
       - ALLSUBSTS contains the substs in the form NAME = VALUE, in sorted
         order, for use in autoconf.mk. It includes ACDEFINES, but doesn't
         include ALLDEFINES. Only substs with a VALUE are included, such that
         the resulting file doesn't change when new empty substs are added.
         This results in less invalidation of build dependencies in the case
         of autoconf.mk..
+      - ALLEMPTYSUBSTS contains the substs with an empty value, in the form
+        NAME =.
 
     ConfigEnvironment expects a "top_srcdir" subst to be set with the top
     source directory, in msys format on windows. It is used to derive a
@@ -113,6 +115,8 @@ class ConfigEnvironment(object):
             shell_escape(self.defines[name])) for name in global_defines])
         self.substs['ALLSUBSTS'] = '\n'.join(sorted(['%s = %s' % (name,
             self.substs[name]) for name in self.substs if self.substs[name]]))
+        self.substs['ALLEMPTYSUBSTS'] = '\n'.join(sorted(['%s =' % name
+            for name in self.substs if not self.substs[name]]))
         self.substs['ALLDEFINES'] = '\n'.join(sorted(['#define %s %s' % (name,
             self.defines[name]) for name in global_defines]))
 
diff --git a/python/mozbuild/mozbuild/test/backend/test_configenvironment.py b/python/mozbuild/mozbuild/test/backend/test_configenvironment.py
index a93ac9495cc7..dd202a3b2907 100644
--- a/python/mozbuild/mozbuild/test/backend/test_configenvironment.py
+++ b/python/mozbuild/mozbuild/test/backend/test_configenvironment.py
@@ -22,14 +22,14 @@ class ConfigEnvironment(ConfigStatus.ConfigEnvironment):
 
 class TestEnvironment(unittest.TestCase):
     def test_auto_substs(self):
-        '''Test the automatically set values of ACDEFINES, ALLDEFINES
-        and ALLSUBSTS.
+        '''Test the automatically set values of ACDEFINES, ALLDEFINES,
+        ALLSUBSTS and ALLEMPTYSUBSTS.
         '''
         env = ConfigEnvironment('.', '.',
                   defines = [ ('foo', 'bar'), ('baz', 'qux 42'),
                               ('abc', 'def'), ('extra', 'foobar') ],
                   non_global_defines = ['extra', 'ignore'],
-                  substs = [ ('FOO', 'bar'), ('ABC', 'def'),
+                  substs = [ ('FOO', 'bar'), ('FOOBAR', ''), ('ABC', 'def'),
                              ('bar', 'baz qux'), ('zzz', '"abc def"'),
                              ('qux', '') ])
         # non_global_defines should be filtered out in ACDEFINES and
@@ -47,6 +47,9 @@ ACDEFINES = -Dfoo=bar -Dbaz=qux\ 42 -Dabc=def
 FOO = bar
 bar = baz qux
 zzz = "abc def"''')
+        # ALLEMPTYSUBSTS contains all substs with no value.
+        self.assertEqual(env.substs['ALLEMPTYSUBSTS'], '''FOOBAR =
+qux =''')
 
     def test_config_file(self):
         '''Test the creation of config files.