Fixup for bug 911634. r=gps

client.mk

@@ -201,8 +201,9 @@ endif
 
 ifdef WANT_MOZCONFIG_MK
 # For now, only output "export" lines from mozconfig2client-mk output.
+MOZCONFIG_MK_LINES := $(filter export||%,$(MOZCONFIG_OUT_LINES))
 $(OBJDIR)/.mozconfig.mk: $(FOUND_MOZCONFIG) $(call mkdir_deps,$(OBJDIR))
-	( $(foreach line,$(filter export||%,$(MOZCONFIG_OUT_LINES)), echo "$(subst ||, ,$(line))";) ) > $@
+	$(if $(MOZCONFIG_MK_LINES),( $(foreach line,$(MOZCONFIG_MK_LINES), echo "$(subst ||, ,$(line))";) )) > $@
 
 # Include that makefile so that it is created. This should not actually change
 # the environment since MOZCONFIG_CONTENT, which MOZCONFIG_OUT_LINES derives