mu: generate mu4e-autoloads.el

* What is autoload?

Basically, it allows you to delay the loading of a package[1].

* What is a name-autoloads.el file?

When installing a package using package.el, Emacs generates[1] a
name-autoloads.el file for that package, which is used to autoload the
principal user commands defined[2][3] in the package.

* What problem does this patch solve?

There are some autoload commands defined[3] by mu4e.  Before, those
autoload commands cannot be autoloaded because of the missing
mu4e-autoloads.el file since mu4e installed from nixpkgs does not use
package.el.  This patch fixes that.

To sum up, with this patch, you can call (mu4e) without
calling (require 'mu4e) first.

* Isn't it better to contribute to the build system of the upstream mu
project to generate that mu4e-autoloads.el file?

It seems impossible[4] to do so.

[1]: https://www.gnu.org/software/emacs/manual/html_node/elisp/Autoload.html
[2]: https://www.gnu.org/software/emacs/manual/html_node/elisp/Packaging-Basics.html
[3]: 581f8d7e92/mu4e/mu4e.el (L58)
[4]: https://github.com/djcb/mu/pull/2313

pkgs/tools/networking/mu/default.nix

@@ -40,6 +40,8 @@ stdenv.mkDerivation rec {
     find $out/share/emacs -type f -name '*.el' -print0 \
       | xargs -0 -I {} -n 1 -P $NIX_BUILD_CORES sh -c \
           "emacs --batch --eval '(setq large-file-warning-threshold nil)' -f batch-native-compile {} || true"
+  '' + ''
+    emacs --batch -l package --eval "(package-generate-autoloads \"mu4e\" \"$out/share/emacs/site-lisp/mu4e\")"
   '';
 
   buildInputs = [ emacs glib gmime3 texinfo xapian ];