This makes it easier to detect support for various features at compile
time.

Enables: https://github.com/lxc/go-lxc/pull/149
Fixes: https://launchpadlibrarian.net/526273274/buildlog_snap_ubuntu_bionic_i386_lxd-4.0-edge_BUILDING.txt.gzSigned-off-by: Christian Brauner <christian.brauner@ubuntu.com>