If building libvirt against Ubuntu precise, the librbd.h is lacking
many functions that libvirt expects. We have no version check, so
we were enabling RBD even though it cannot compile. This configure
check uses existance of 'rbd_get_features' as an identifier for the
min required version.
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
LIBVIRT_ARG_WITH_ALT is more generic than LIBVIRT_ARG_WITH, which
is tailored at switching features on and off.
Rename the macros according to their intended purpose, and add
some documentation to help developers pick between the two.
