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.