bug#71627: Guix-packaged emacs does not work with alpha-background frame parameter

[bdunahu]

Hi!

I am running into an issue on most variants of the GNU Emacs package in
that the alpha-background frame parameter, a feature of emacs 29.1 as
described here:

https://www.emacswiki.org/emacs/TransparentEmacs

does nothing, rather than changing the frame transparency as intended.
The (similar) 'alpha' frame-parameter described in the same source above
does work.

This issue exists with the 'emacs', 'emacs-next', and related packages
for Xorg. Interestingly, the 'emacs-pgtk' package does work as intended,
even when used on Xorg. The feature also works correctly when emacs
without pure-gtk support is installed through alternative package
managers.

My best guess is that a required compile flag is missing for the feature
to work. I am not yet experienced enough with guix to see if I can
create a package declaration that does not have this issue.

Thank you if you decide to look into it!
bd