+## Enable TLS-PSK (preshared keys).
+AH_TEMPLATE([RS_ENABLE_TLS_PSK], [TLS-PSK (TLS preshared keys) enabled])
+AC_ARG_ENABLE([tls-psk], AS_HELP_STRING([--enable-tls-psk], [enable TLS-PSK (TLS preshared keys)]),
+ [AC_CHECK_LIB([ssl], [SSL_set_psk_client_callback],,
+ AC_MSG_ERROR([required library openssl with SSL_set_psk_client_callback() not found]))
+ AC_DEFINE([RS_ENABLE_TLS_PSK])])
+AM_CONDITIONAL([RS_ENABLE_TLS_PSK], [test "${enable_tls_psk+set}" = set])