emacs: enable strictDeps for elisp packages

This is a step towards the goal[1] of enabling strictDeps by default
in Nixpkgs.

This helps users write better buildInputs/nativeBuildinputs and
checkInputs/nativeCheckInputs.  It also makes the build faster because
it reduces unnecessary runs of hooks added by addEnvHooks.

Previously, emacs was only in nativeBuildInputs.  After strictDeps is
enabled, emacs has to be also in buildInputs to provide a header file
needed[2] by dynamic modules from an elisp package.  Only some elisp
packages contain dynamic modules but we always add emacs to
buildInputs since there is no harm.

[1]: https://github.com/NixOS/nixpkgs/issues/178468
[2]: info "(elisp) Module Initialization"

pkgs/applications/editors/emacs/build-support/generic.nix

@@ -67,7 +67,7 @@ libBuildHelper.extendMkDerivation' stdenv.mkDerivation (
       '';
 
     inherit packageRequires;
-    buildInputs = finalAttrs.packageRequires ++ buildInputs;
+    buildInputs = [ emacs ] ++ finalAttrs.packageRequires ++ buildInputs;
     nativeBuildInputs = [
       emacs
       texinfo
@@ -77,6 +77,8 @@ libBuildHelper.extendMkDerivation' stdenv.mkDerivation (
 
     setupHook = args.setupHook or setupHook;
 
+    strictDeps = args.strictDeps or true;
+
     inherit turnCompilationWarningToError ignoreCompilationError;
 
     meta =