axTLS: not considered fit for use

URL: https://curl.haxx.se/mail/lib-2018-06/0000.html

This is step one. It adds #error statements that require source edits to
make curl build again if asked to use axTLS. At a later stage we might
remove the axTLS specific code completely.

Closes #2628

lib/vtls/axtls.c

@@ -29,6 +29,11 @@
 #include "curl_setup.h"
 
 #ifdef USE_AXTLS
+
+#error axTLS support has been disabled in curl due to doubts about quaility,
+#error user dedication and a lack of use/testing. We urge users to consider
+#error using a more established TLS backend instead.
+
 #include <axTLS/config.h>
 #include <axTLS/ssl.h>
 #include "axtls.h"