krb5: fix detecting channel binding feature

Use the already detected `gssapi/gssapi_krb5.h` MIT Kerberos header
to pull in `gssapi_ext.h`, which in turn sets `GSS_C_CHANNEL_BOUND_FLAG`
if supported. Channel binding is present in MIT Kerberos 1.19+.

Also:
- lib: de-duplicate GSS-API header includes.
- vauth: de-duplicate `urldata.h` includes.
- drop interim feature macro in favor of the native GSS one.

Assisted-by: Max Faxälv
Reported-by: Max Faxälv
Bug: https://github.com/curl/curl/pull/19164#issuecomment-3551687025
Follow-up to 8616e5aada #19164
Closes #19603
Closes #19760

lib/vauth/vauth.h

@@ -28,6 +28,7 @@
 
 #include "../bufref.h"
 #include "../curlx/dynbuf.h"
+#include "../urldata.h"
 
 struct Curl_easy;
 struct connectdata;
@@ -233,18 +234,6 @@ CURLcode Curl_auth_create_xoauth_bearer_message(const char *user,
 
 #ifdef USE_KERBEROS5
 
-#ifdef HAVE_GSSAPI
-# ifdef HAVE_GSSGNU
-#  include <gss.h>
-# else
-#  ifdef HAVE_GSSAPI_H
-#   include <gssapi.h>
-#  else
-#   include <gssapi/gssapi.h>
-#  endif
-# endif
-#endif
-
 /* meta key for storing KRB5 meta at connection */
 #define CURL_META_KRB5_CONN   "meta:auth:krb5:conn"
 
@@ -310,7 +299,7 @@ struct negotiatedata {
   gss_ctx_id_t context;
   gss_name_t spn;
   gss_buffer_desc output_token;
-#ifdef CURL_GSSAPI_HAS_CHANNEL_BINDING
+#ifdef GSS_C_CHANNEL_BOUND_FLAG
   struct dynbuf channel_binding_data;
 #endif
 #else