emacs: Clear the environment while dumping

This reduces the closure size of Emacs from 575 to 279 MiB. Dumping
Emacs had a chance of leaking parts of the environment (such as $PATH)
into the dumped executable. This hopefully fixes it. (It's a bit hard
to tell since the effect is not deterministic.)

pkgs/applications/editors/emacs/clean-env.patch

@@ -0,0 +1,15 @@
+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 -l loadup dump
++	env -i LC_ALL=C $(RUN_TEMACS) -batch -l loadup dump
+   ifneq ($(PAXCTL_dumped),)
+ 	$(PAXCTL_dumped) $@
+   endif

pkgs/applications/editors/emacs/default.nix

@@ -34,7 +34,11 @@ stdenv.mkDerivation rec {
     sha256 = "02y00y9q42g1iqgz5qhmsja75hwxd88yrn9zp14lanay0zkwafi5";
   };
 
-  patches = (lib.optional stdenv.isDarwin ./at-fdcwd.patch);
+  enableParallelBuilding = true;
+
+  patches =
+    [ ./clean-env.patch ]
+    ++ lib.optional stdenv.isDarwin ./at-fdcwd.patch;
 
   nativeBuildInputs = [ pkgconfig ]
     ++ lib.optionals srcRepo [ autoconf automake texinfo ]