bug#38261: Recent changes to emacs build system

[brettg@posteo.net]

Hey all, the recent changes to the emacs build system result in a few 
broken packages like emacs-pdf-tools, or basically anything that uses a 
phase for emacs-set-emacs-load-path.

For example, I borrowed the technique used by emacs-pdf-tools to package 
emacs-telega, resulting in both packages failing to build:

Here is the code for emacs-telega:

https://git.sr.ht/~brettgilio/cfg/tree/master/channel/non-gnu/packages/emacs-xyz.scm#L99

The issue is in this phase for both emacs-pdf-tools and my channel 
package:

  (add-after 'compress-documentation 'emacs-set-emacs-load-path
				  (assoc-ref emacs:%standard-phases 'set-emacs-load-path))

Resulting in:

starting phase `emacs-set-emacs-load-path'
Backtrace:
            5 (primitive-load "/gnu/store/5b1p1gsvfyi4fbx4s42rhab2dns…")
In ice-9/eval.scm:
    191:35  4 (_ _)
In ice-9/boot-9.scm:
     829:9  3 (catch _ _ #<procedure 7ffff3bbb518 at /gnu/store/zmkg…> …)
In srfi/srfi-1.scm:
    863:16  2 (every1 #<procedure 7ffff30ae160 at /gnu/store/zmkgrvv…> …)
In 
/gnu/store/zmkgrvvhmrix2b1z7id6zrg9bb7qxzdl-module-import/guix/build/gnu-build-system.scm:
    839:30  1 (_ _)
In unknown file:
            0 (_ #:source "/gnu/store/qw8xbmk6ryl9a2jrp0gip3yffmsdix…" …)

ERROR: Wrong type to apply: #f

If we suspect that changes are going to be non-backwards compatible 
could we use the news system to pass along that message? Much 
appreciated. Thanks.

Brett Gilio