threads: support libgcrypt 1.6 hack

Not 100% satisfied of this patch, but the way libgcrypt handles
threading in 1.6 is not compatible with custom handlers. The
new code basicaly uses pthreads in every case. This will probably
not work on windows.

doc/threading.dox

@@ -61,5 +61,6 @@ implement the following methods :
 - mutex_destroy
 - thread_id
 
+libgcrypt 1.6 and bigger backend does not support custom callback. Using anything else than pthreads (ssh_threads_get_pthread()) here will fail.
 Good luck !
 */

include/libssh/callbacks.h

@@ -801,6 +801,8 @@ struct ssh_threads_callbacks_struct {
  *
  * @see ssh_threads_callbacks_struct
  * @see SSH_THREADS_PTHREAD
+ * @bug libgcrypt 1.6 and bigger backend does not support custom callback.
+ *      Using anything else than pthreads here will fail.
  */
 LIBSSH_API int ssh_threads_set_callbacks(struct ssh_threads_callbacks_struct
     *cb);

src/threads.c

@@ -59,8 +59,28 @@ struct ssh_threads_callbacks_struct *ssh_threads_get_noop(void) {
 static struct ssh_threads_callbacks_struct *user_callbacks =&ssh_threads_noop;
 
 #ifdef HAVE_LIBGCRYPT
+#if (GCRYPT_VERSION_NUMBER >= 0x010600)
+/* libgcrypt >= 1.6 does not support custom callbacks */
+GCRY_THREAD_OPTION_PTHREAD_IMPL;
 
-/* Libgcrypt specific way of handling thread callbacks */
+static int libgcrypt_thread_init(void){
+	if(user_callbacks == NULL)
+		return SSH_ERROR;
+	if(user_callbacks == &ssh_threads_noop)
+		return SSH_OK;
+	if (strcmp(user_callbacks->type, "threads_pthread") == 0){
+		gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
+		return SSH_OK;
+	} else {
+		/* not supported */
+		SSH_LOG(SSH_LOG_WARN, "Custom thread handlers not supported with libgcrypt >=1.6, using pthreads");
+		gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
+		return SSH_OK;
+	}
+}
+
+#else
+/* Libgcrypt < 1.6 specific way of handling thread callbacks */
 
 static struct gcry_thread_cbs gcrypt_threads_callbacks;
 
@@ -79,7 +99,8 @@ static int libgcrypt_thread_init(void){
 	gcry_control(GCRYCTL_SET_THREAD_CBS, &gcrypt_threads_callbacks);
 	return SSH_OK;
 }
-#else
+#endif /* GCRYPT_VERSION_NUMBER */
+#else /* HAVE_LIBGCRYPT */
 
 /* Libcrypto specific stuff */