Dump temacs in an empty environment to prevent -dev paths from ending
up in the dumped image.

diff -ru -x '*~' emacs-25.3/src/Makefile.in emacs-25.3-new/src/Makefile.in
--- emacs-25.3/src/Makefile.in	2017-04-14 17:02:47.000000000 +0200
+++ emacs-25.3-new/src/Makefile.in	2017-09-25 19:03:02.173861038 +0200
@@ -532,7 +532,7 @@
 ifeq ($(CANNOT_DUMP),yes)
 	ln -f temacs$(EXEEXT) $@
 else
-	LC_ALL=C $(RUN_TEMACS) -batch $(BUILD_DETAILS) -l loadup dump
+	env -i LC_ALL=C $(RUN_TEMACS) -batch $(BUILD_DETAILS) -l loadup dump
   ifneq ($(PAXCTL_dumped),)
 	$(PAXCTL_dumped) $@
   endif