enchant: explicitly enable required providers

leaving provider detection to the build process is error-prone, as a
missing or invalid dependency will just result in the derivation being
built without it enabled.

As of now, the nuspell provider is not buildable:

    configure: error: --with-nuspell was given, but test(s) for nuspell failed

pkgs/development/libraries/enchant/2.x.nix

@@ -46,6 +46,10 @@ stdenv.mkDerivation rec {
 
   configureFlags = [
     "--enable-relocatable" # needed for tests
+    "--with-aspell"
+    "--with-hspell"
+    "--with-hunspell"
+    "--with-nuspell"
   ];
 
   meta = with lib; {