b5-enter: support building without unshare cgroup support

The current version of barney won't compile on the user servers
because they don't have CLONE_NEWCGROUP defined. This change provides
an option to compile a version of barney that doesn't need cgroup
support.

The default make operation will build without cgroup support. To build
a version with cgroup support, the syntax is:
sudo make install CFLAGS=-DHAVE_CLONE_NEWCGROUP=1

Change-Id: Icbc20bbe8e287ff298dba4563216fa54466b2827

enter.c

@@ -33,9 +33,11 @@ static int opts_to_unshareflags(const struct entry_settings *opts)
 	if (opts->mount) {
 		flags |= CLONE_NEWNS;
 	}
+#ifdef HAVE_CLONE_NEWCGROUP
 	if (opts->cgroup) {
 		flags |= CLONE_NEWCGROUP;
 	}
+#endif
 	if (opts->ipc) {
 		flags |= CLONE_NEWIPC;
 	}