don't build init.lxc.static if libcap.a isn't available

Note that building init.lxc.static still requires a static libutil.a
and libpthread.a, but these are available on most distro's through
glibc-static.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>