emacs: prepare webp support for versions >=29.1

This prepares the derivation for the upcoming webp image file format
support with Emacs 29.1. Therefore this change is mainly useful with the
"emacs-overlay".

https://github.com/nix-community/emacs-overlay/

Changelog from upstream Emacs:

https://github.com/emacs-mirror/emacs/blob/1cdb8d4c99bf82579dd180187a11f2bcbeccd4c4/etc/NEWS#L435-L439

More info on image formats in Emacs and how to check for their support:

https://www.gnu.org/software/emacs/manual/html_node/elisp/Image-Formats.html

pkgs/applications/editors/emacs/generic.nix

@@ -8,7 +8,7 @@
 }:
 { stdenv, lib, fetchurl, fetchpatch, ncurses, xlibsWrapper, libXaw, libXpm
 , Xaw3d, libXcursor,  pkg-config, gettext, libXft, dbus, libpng, libjpeg, giflib
-, libtiff, librsvg, gconf, libxml2, imagemagick, gnutls, libselinux
+, libtiff, librsvg, libwebp, gconf, libxml2, imagemagick, gnutls, libselinux
 , alsa-lib, cairo, acl, gpm, AppKit, GSS, ImageIO, m17n_lib, libotf
 , sigtool, jansson, harfbuzz, sqlite, nixosTests
 , dontRecurseIntoAttrs, emacsPackagesFor
@@ -22,6 +22,7 @@
 , withMotif ? false, motif ? null
 , withSQLite3 ? false
 , withCsrc ? true
+, withWebP ? false
 , srcRepo ? false, autoreconfHook ? null, texinfo ? null
 , siteStart ? ./site-start.el
 , nativeComp ? false
@@ -134,6 +135,7 @@ let emacs = stdenv.mkDerivation (lib.optionalAttrs nativeComp {
     ++ lib.optionals (withX && withGTK3) [ gtk3-x11 gsettings-desktop-schemas ]
     ++ lib.optional (withX && withMotif) motif
     ++ lib.optional withSQLite3 sqlite
+    ++ lib.optional withWebP libwebp
     ++ lib.optionals (withX && withXwidgets) [ webkitgtk glib-networking ]
     ++ lib.optionals withNS [ AppKit GSS ImageIO ]
     ++ lib.optionals stdenv.isDarwin [ sigtool ]