gnutls: don't break if ALPN extension is not available

At least, fedora 20 doesn't enable it.

modules/misc/gnutls.c

@@ -198,6 +198,7 @@ static int gnutls_SessionOpen (vlc_tls_t *tls, int type,
         goto error;
     }
 
+#ifdef GNUTLS_ALPN_MAND
     if (alpn != NULL)
     {
         gnutls_datum_t *protv = NULL;
@@ -222,6 +223,9 @@ static int gnutls_SessionOpen (vlc_tls_t *tls, int type,
         val = gnutls_alpn_set_protocols (session, protv, protc, 0);
         free (protv);
     }
+#else
+    VLC_UNUSED(alpn);
+#endif
 
     gnutls_transport_set_int (session, fd);
 
@@ -275,6 +279,7 @@ static int gnutls_ContinueHandshake (vlc_tls_t *tls, char **restrict alp)
     return -1;
 
 done:
+#ifdef GNUTLS_ALPN_MAND
     if (alp != NULL)
     {
         gnutls_datum_t datum;
@@ -292,6 +297,9 @@ done:
         else
             *alp = NULL;
     }
+#else
+    VLC_UNUSED(alp);
+#endif
     return 0;
 }