configure: Add a relay module, with --disable-module-relay

The Makefile and C defines don't actually do anything yet.

Part of 32123.

configure.ac

@@ -255,7 +255,16 @@ dnl Tor modules options. These options are namespaced with --disable-module-XXX
 dnl ---
 
 dnl All our modules.
-m4_define(MODULES, dirauth)
+m4_define(MODULES, relay dirauth)
+
+dnl Relay module.
+AC_ARG_ENABLE([module-relay],
+              AS_HELP_STRING([--disable-module-relay],
+                             [Build tor without the Relay modules: tor can not run as a relay, bridge, or authority]))
+AM_CONDITIONAL(BUILD_MODULE_RELAY, [test "x$enable_module_relay" != "xno"])
+AM_COND_IF(BUILD_MODULE_RELAY,
+           AC_DEFINE([HAVE_MODULE_RELAY], [1],
+                     [Compile with Relay feature support]))
 
 dnl Directory Authority module.
 AC_ARG_ENABLE([module-dirauth],