Store Git commit identifier in its own object

Fixes #1640

src/version.c

@@ -0,0 +1,11 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009-2015 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * Stores the latest Git commit identifier so that it can be linked into i3
+ * and used dynamically without recompiling every object file.
+ *
+ */
+const char *i3_version = I3_VERSION;