Configurations/gentemplate.pm: Generate generators too, when necessary

A generator in a `GENERATE[generated]=generator` build.info statement may
itself be generated.  That needs to be taken into account.

This was always meant to be, but we missed the spot, for lack of use cases.
Now we have one.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18263)

Configurations/gentemplate.pm

@@ -227,6 +227,10 @@ sub dogenerate {
         foreach (@{$self->{info}->{depends}->{$src} // []}) {
             $self->dogenerate($_, $obj, $bin, %opts);
         }
+        # The generator itself may be is generated
+        if ($self->{info}->{generate}->{$script}) {
+            $self->dogenerate($script, $obj, $bin, %opts);
+        }
     }
     $cache{$src} = 1;
 }