gnu: emacs: Do not keep references to the entries of the build-time $PATH.

This reduces the number of references of the output from 54 to 31,
removing references notably to gcc, expat, glib:bin, ld-wrapper-0,
texinfo, pkg-config, make, gawk, binutils, etc.

Reported by David Thompson.

* gnu/packages/patches/emacs-exec-path.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/emacs.scm (emacs)[source]: Use it.

gnu-system.am

@@ -407,6 +407,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/doxygen-tmake.patch			\
   gnu/packages/patches/duplicity-piped-password.patch		\
   gnu/packages/patches/duplicity-test_selection-tmp.patch	\
+  gnu/packages/patches/emacs-exec-path.patch			\
   gnu/packages/patches/eudev-rules-directory.patch		\
   gnu/packages/patches/findutils-absolute-paths.patch		\
   gnu/packages/patches/flashrom-use-libftdi1.patch		\

gnu/packages/emacs.scm

@@ -64,7 +64,8 @@
                                  version ".tar.xz"))
              (sha256
               (base32
-               "0kn3rzm91qiswi0cql89kbv6mqn27rwsyjfb8xmwy9m5s8fxfiyx"))))
+               "0kn3rzm91qiswi0cql89kbv6mqn27rwsyjfb8xmwy9m5s8fxfiyx"))
+             (patches (list (search-patch "emacs-exec-path.patch")))))
     (build-system glib-or-gtk-build-system)
     (arguments
      '(#:phases (alist-cons-before

gnu/packages/patches/emacs-exec-path.patch

@@ -0,0 +1,18 @@
+Do not capture the build-time value of $PATH in the 'emacs' executable
+since this can noticeably increase the size of the closure of Emacs
+with things like GCC being referenced.
+
+--- a/lisp/loadup.el
++++ b/lisp/loadup.el
+@@ -64,6 +64,11 @@
+ 			    (expand-file-name "textmodes" dir)
+ 			    (expand-file-name "vc" dir)))))
+ 
++;; Reset 'exec-path' so we don't capture the build-time $PATH in the
++;; 'emacs' executable.
++(setq-default exec-path nil)
++(setq exec-path nil)
++
+ (if (eq t purify-flag)
+     ;; Hash consing saved around 11% of pure space in my tests.
+     (setq purify-flag (make-hash-table :test 'equal :size 70000)))