Add autotools support for BtrFS option

Feature is enabled by default, if headers are available. It can be
turned off explictly.

src/usermod.c

@@ -1819,12 +1819,14 @@ static void move_home (void)
 			return;
 		} else {
 			if (EXDEV == errno) {
+#ifdef WITH_BTRFS
 				if (btrfs_is_subvolume (prefix_user_home) > 0) {
 					fprintf (stderr,
 					        _("%s: error: cannot move subvolume from %s to %s - different device\n"),
 					        Prog, prefix_user_home, prefix_user_newhome);
 					fail_exit (E_HOMEDIR);
 				}
+#endif
 
 				if (copy_tree (prefix_user_home, prefix_user_newhome, true,
 				               true,