Allow Emacs on w32 to build against and work with GnuTLS 3.x.

 src/gnutls.c (init_gnutls_functions): Load gnutls_check_version.
 Load gnutls_transport_set_lowat only if GnuTLS version is below
 2.11.1.
 (emacs_gnutls_handshake): Call gnutls_transport_set_lowat only for
 GnuTLS versions below 2.11.1.
 lisp/term/w32-win.el (dynamic-library-alist) <gnutls>: Load
 libgnutls-28.dll, from GnuTLS version 3.x, in preference to
 libgnutls-26.dll.

lisp/ChangeLog

+2012-01-01  Eli Zaretskii  <eliz@gnu.org>
+
+	* term/w32-win.el (dynamic-library-alist) <gnutls>: Load
+	libgnutls-28.dll, from GnuTLS version 3.x, in preference to
+	libgnutls-26.dll.
+
 2011-12-31  Andreas Schwab  <schwab@linux-m68k.org>
 
 	* emacs-lisp/bytecomp.el (byte-compile-file): Fix indentation.

lisp/term/w32-win.el

@@ -210,7 +210,7 @@ See the documentation of `create-fontset-from-fontset-spec' for the format.")
        '(gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
        '(glib "libglib-2.0-0.dll")
        '(gobject "libgobject-2.0-0.dll")
-       '(gnutls "libgnutls-26.dll")))
+       '(gnutls "libgnutls-28.dll" "libgnutls-26.dll")))
 
 ;;; multi-tty support
 (defvar w32-initialized nil

src/ChangeLog

+2012-01-01  Eli Zaretskii  <eliz@gnu.org>
+
+	* gnutls.c (init_gnutls_functions): Load gnutls_check_version.
+	Load gnutls_transport_set_lowat only if GnuTLS version is below
+	2.11.1.
+	(emacs_gnutls_handshake): Call gnutls_transport_set_lowat only for
+	GnuTLS versions below 2.11.1.
+
 2011-12-31  Antoine Levitt  <antoine.levitt@gmail.com>
 
 	* xdisp.c (syms_of_xdisp) <window-scroll-functions>: Add warning

src/gnutls.c

@@ -125,6 +125,7 @@ DEF_GNUTLS_FN (ssize_t, gnutls_record_send,
 	       (gnutls_session_t, const void *, size_t));
 DEF_GNUTLS_FN (const char *, gnutls_strerror, (int));
 DEF_GNUTLS_FN (void, gnutls_transport_set_errno, (gnutls_session_t, int));
+DEF_GNUTLS_FN (const char *, gnutls_check_version, (const char *));
 DEF_GNUTLS_FN (void, gnutls_transport_set_lowat, (gnutls_session_t, int));
 DEF_GNUTLS_FN (void, gnutls_transport_set_ptr2,
 	       (gnutls_session_t, gnutls_transport_ptr_t,
@@ -184,7 +185,11 @@ init_gnutls_functions (Lisp_Object libraries)
   LOAD_GNUTLS_FN (library, gnutls_record_send);
   LOAD_GNUTLS_FN (library, gnutls_strerror);
   LOAD_GNUTLS_FN (library, gnutls_transport_set_errno);
-  LOAD_GNUTLS_FN (library, gnutls_transport_set_lowat);
+  LOAD_GNUTLS_FN (library, gnutls_check_version);
+  /* We don't need to call gnutls_transport_set_lowat in GnuTLS 2.11.1
+     and later, and the function was removed entirely in 3.0.0.  */
+  if (!fn_gnutls_check_version ("2.11.1"))
+    LOAD_GNUTLS_FN (library, gnutls_transport_set_lowat);
   LOAD_GNUTLS_FN (library, gnutls_transport_set_ptr2);
   LOAD_GNUTLS_FN (library, gnutls_transport_set_pull_function);
   LOAD_GNUTLS_FN (library, gnutls_transport_set_push_function);
@@ -282,7 +287,12 @@ emacs_gnutls_handshake (struct Lisp_Process *proc)
 	 (Note: this is probably not strictly necessary as the lowat
 	  value is only used when no custom pull/push functions are
 	  set.)  */
-      fn_gnutls_transport_set_lowat (state, 0);
+      /* According to GnuTLS NEWS file, lowat level has been set to
+	 zero by default in version 2.11.1, and the function
+	 gnutls_transport_set_lowat was removed from the library in
+	 version 2.99.0.  */
+      if (!fn_gnutls_check_version ("2.11.1"))
+	fn_gnutls_transport_set_lowat (state, 0);
 #else
       /* This is how GnuTLS takes sockets: as file descriptors passed
 	 in.  For an Emacs process socket, infd and outfd are the