font-manager: allow building without webkit

webkit is only beneficial if browsing non-local fonts and building
without it greatly reduces closure size (measurements from
`nix path-info -S` on x86_64):

- `withWebkit=true`: 1203033312
- `withWebkit=false`: 372203760

upstream calls this feature "Google Font integration" here:
- <https://github.com/FontManager/font-manager#building-from-source>

but it's controlled via the "webkit" meson flag:
- <https://github.com/FontManager/font-manager/blob/master/meson.build#L29>

pkgs/applications/misc/font-manager/default.nix

···
 , desktop-file-utils
 , wrapGAppsHook
 , gobject-introspection
-, libsoup
-, glib-networking
-, webkitgtk
+# withWebkit enables the "webkit" feature, also known as Google Fonts
+, withWebkit ? true, glib-networking, libsoup, webkitgtk
 }:
 
 stdenv.mkDerivation rec {
···
     gsettings-desktop-schemas # for font settings
     gtk3
     gnome.adwaita-icon-theme
+  ] ++ lib.optionals withWebkit [
+    glib-networking # for SSL so that Google Fonts can load
     libsoup
-    glib-networking # for SSL so that Google Fonts can load
     webkitgtk
   ];
 
   mesonFlags = [
     "-Dreproducible=true" # Do not hardcode build directory…
+    (lib.mesonBool "webkit" withWebkit)
   ];
 
   postPatch = ''