lib: allow disabling optional libraries

Add support for the disable_libs option, to allow disabling the build of
particular libraries. As part of this, maintain a list of what libraries
can safely be disabled, without breaking the build - for now this list is
solely those libraries which are not built on FreeBSD, kni, power and
vhost. This list can be expanded by future patches.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>