http://curl.haxx.se/mail/lib-2011-10/0176.html

Curl: [PATCH] gtls: only call gnutls_transport_set_lowat with <gnutls-2.12.0
From: Tim Harder <radhermit_at_gmail.com>
Date: Wed, 19 Oct 2011 01:08:56 -0700

The default lowat level for gnutls-2.12* is set to zero to avoid
unnecessary system calls and the gnutls_transport_set_lowat function has
been totally removed in >=gnutls-3 which causes build failures.
Therefore, the function shouldn't be used except for versions that
require it, <gnutls-2.12.0.

---
 lib/gtls.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/lib/gtls.c b/lib/gtls.c
index f75a815..7ca46c8 100644
--- a/lib/gtls.c
+++ b/lib/gtls.c
@@ -476,8 +476,10 @@ gtls_connect_step1(struct connectdata *conn,
   gnutls_transport_set_push_function(session, Curl_gtls_push);
   gnutls_transport_set_pull_function(session, Curl_gtls_pull);
 
+#if GNUTLS_VERSION_NUMBER < 0x020c00
   /* lowat must be set to zero when using custom push and pull functions. */
   gnutls_transport_set_lowat(session, 0);
+#endif
 
   /* This might be a reconnect, so we check for a session ID in the cache
      to speed up things */ -- 
1.7.7