In the effort to make LXC work with non-standard Linux distros, this change
allows for the user to build LXC without capability support through a new
--disable-capabilities option to configure.

This effectively will cause LXC not to link against libcap and will turn all
the _cap_ functions into no-ops.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>