Cumbersome indeed.  I also thought of installing to a local "output"
folder and using this as source.  The issue is that the prefix (as in
"./configure --prefix=") must be set to the store output of emacs-dev.
Otherwise Emacs won't find its Lisp folders.

I can't think of a clean way to do it.

Let's go back to my original intention: I want to do is a clean way to
install work-in-progress projects.  On traditional Unices, this can be
done by calling `make install` to /usr/local (possibly with the help of
`stow`).

On Guix, installing to /usr/local won't help: in all cases we've got to
set all the *PATH variables to folders of the locally compiled project.
I initially thought that the Guix package manager could help but it
seems that I was wrong: Guix is made to _build_ packages reproducibly,
not to install some random files.

Now I'm thinking that a better approach to this issue is to install,
say, in ~/.local and to set all PATH variables appropriately, e.g.

--8<---------------cut here---------------start------------->8---
export C_INCLUDE_PATH="$HOME/.local/include":$C_INCLUDE_PATH
export CPLUS_INCLUDE_PATH="$HOME/.local/include":$CPLUS_INCLUDE_PATH
export LIBRARY_PATH="$HOME/.local/lib":$LIBRARY_PATH
export PKG_CONFIG_PATH="$HOME/.local/lib/pkgconfig":$PKG_CONFIG_PATH
export INFOPATH="$HOME/.local/share/info":$INFOPATH
export MANPATH="$HOME/.local/share/man":$MANPATH
# ...
--8<---------------cut here---------------end--------------->8---

Lots of work ahead! :p

For emacs, maybe a better way is to simply add
~/projects/emacs/src/emacs to the PATH.

Does anyone know of a better approach?

--
Pierre Neidhardt