emacs: make ImageMagick dependency optional in Emacs≥27

Emacs 27 doesn't use ImageMagick by default to display images [1]:

    ** Emacs no longer defaults to using ImageMagick to display images.
    This is due to security and stability concerns with ImageMagick.  To
    override the default, use 'configure --with-imagemagick'.

[1] https://github.com/emacs-mirror/emacs/blob/emacs-27.1/etc/NEWS#L96-L98

pkgs/applications/editors/emacs/generic.nix

@@ -22,6 +22,7 @@
 , srcRepo ? false, autoreconfHook ? null, texinfo ? null
 , siteStart ? ./site-start.el
 , nativeComp ? false
+, withImageMagick ? lib.versionOlder version "27" && (withX || withNS)
 , toolkit ? (
   if withGTK2 then "gtk2"
   else if withGTK3 then "gtk3"
@@ -101,7 +102,8 @@ in stdenv.mkDerivation {
     ++ lib.optionals withX
       [ xlibsWrapper libXaw Xaw3d libXpm libpng libjpeg libungif libtiff libXft
         gconf cairo ]
-    ++ lib.optionals (withX || withNS) [ imagemagick librsvg ]
+    ++ lib.optionals (withX || withNS) [ librsvg ]
+    ++ lib.optionals withImageMagick [ imagemagick ]
     ++ lib.optionals (stdenv.isLinux && withX) [ m17n_lib libotf ]
     ++ lib.optional (withX && withGTK2) gtk2-x11
     ++ lib.optionals (withX && withGTK3) [ gtk3-x11 gsettings-desktop-schemas ]
@@ -126,6 +128,7 @@ in stdenv.mkDerivation {
              "--with-gif=no" "--with-tiff=no" ])
     ++ lib.optional withXwidgets "--with-xwidgets"
     ++ lib.optional nativeComp "--with-nativecomp"
+    ++ lib.optional withImageMagick "--with-imagemagick"
     ;
 
   installTargets = [ "tags" "install" ];