kerberos: Disable on darwin

pkgs/servers/openafs-client/default.nix

···
 
     ./regen.sh
 
-    export KRB5_CONFIG=${kerberos}/bin/krb5-config
+    ${optionalString (kerberos != null) ''
+      export KRB5_CONFIG=${kerberos}/bin/krb5-config"
+    ''}
 
     configureFlagsArray=(
       "--with-linux-kernel-build=$TMP/linux"
-      "--with-krb5"
+      ${stdenv.lib.optionalString (kerberos != null) "--with-krb5"}
       "--sysconfdir=/etc/static"
     )
   '';

pkgs/top-level/all-packages.nix

···
   hamlib = callPackage ../development/libraries/hamlib { };
 
   # TODO : Add MIT Kerberos and let admin choose.
-  kerberos = heimdal;
+  # TODO : Fix kerberos on Darwin
+  kerberos = if stdenv.isDarwin then null else heimdal;
 
   heimdal = callPackage ../development/libraries/kerberos/heimdal.nix {
     openldap = openldap.override {