gcrypt: Don't use thread callbacks for newer versions of libgcrypt

According to gcrypt.h these callbacks are not used anymore since
version 1.6 and with clang these actually cause deprecation warnings
that let the build on travis (-Werror) fail.

src/libstrongswan/plugins/gcrypt/gcrypt_plugin.c

@@ -43,10 +43,12 @@ struct private_gcrypt_plugin_t {
 	gcrypt_plugin_t public;
 };
 
+#if GCRYPT_VERSION_NUMBER < 0x010600
 /**
  * Define gcrypt multi-threading callbacks as gcry_threads_pthread
  */
 GCRY_THREAD_OPTION_PTHREAD_IMPL;
+#endif
 
 METHOD(plugin_t, get_name, char*,
 	private_gcrypt_plugin_t *this)
@@ -163,7 +165,9 @@ plugin_t *gcrypt_plugin_create()
 {
 	private_gcrypt_plugin_t *this;
 
+#if GCRYPT_VERSION_NUMBER < 0x010600
 	gcry_control(GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
+#endif
 
 	if (!gcry_check_version(GCRYPT_VERSION))
 	{