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/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)))