settings: Use strongswan.conf used during library initialization for reload

Since 4b670a20 we require an explicit strongswan.conf to re-load configurations.
However, the define was missing in the build, breaking SIGHUP based config
reloading.

Fixes #651.

src/libstrongswan/settings/settings_parser.y

@@ -259,13 +259,11 @@ bool settings_parser_parse_file(section_t *root, char *name)
 	helper->file_include(helper, name);
 	if (!settings_parser_open_next_file(helper))
 	{
-#ifdef STRONGSWAN_CONF
-		if (streq(name, STRONGSWAN_CONF))
+		if (lib->conf && streq(name, lib->conf))
 		{
 			DBG2(DBG_CFG, "failed to open config file '%s'", name);
 		}
 		else
-#endif
 		{
 			DBG1(DBG_CFG, "failed to open config file '%s'", name);
 		}