Add a backend interface version number for eventual future changes.

Note that the interface is still a work in progress and might change
while still keeping the same interface version. This is going to be true
until the first stable release of this project (non beta).

include/backends/backend_version.hpp

@@ -0,0 +1,25 @@
+/* Copyright 2015, 2016, Michele Santullo
+ * This file is part of "dindexer".
+ *
+ * "dindexer" is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * "dindexer" is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with "dindexer".  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef idAA27B58429DB41C2AF53204CC5010E94
+#define idAA27B58429DB41C2AF53204CC5010E94
+
+namespace dindb {
+	constexpr const int g_current_iface_version = 1;
+} //namespace dindb
+
+#endif