Bug 870366 - Part 3: Prohibit PREF_JS_EXPORTS in Makefile.ins. r=gps

CLOBBER

@@ -22,4 +22,4 @@
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Bug 1109248 - This needed a CLOBBER on Windows and OSX.
+Bug 870366 - Blacklisting PREF_JS_EXPORTS in Makefile.ins (because of 852814)

config/baseconfig.mk

@@ -90,6 +90,7 @@ _MOZBUILD_EXTERNAL_VARIABLES := \
   NO_DIST_INSTALL \
   OS_LIBS \
   PARALLEL_DIRS \
+  PREF_JS_EXPORTS \
   PROGRAM \
   PYTHON_UNIT_TESTS \
   RESOURCE_FILES \