configure: add SPNEGO to supported features

Bug: http://curl.haxx.se/bug/view.cgi?id=1343
Reported-by: Michael Osipov

configure.ac

@@ -3403,6 +3403,9 @@ fi
 if test "x$USE_NGHTTP2" = "x1"; then
   SUPPORT_FEATURES="$SUPPORT_FEATURES HTTP2"
 fi
+if test "x$curl_spnego_msg" = "xenabled"; then
+  SUPPORT_FEATURES="$SUPPORT_FEATURES SPNEGO"
+fi
 
 AC_SUBST(SUPPORT_FEATURES)