Some versions of Xen 4.4 have backported 4.5's generation_id fixes

So we detect the specific signature changes in configure, rather than
relying on a coarse-grained version check.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #63 from djs55/flexible-generation-id

Some versions of Xen 4.4 have backported 4.5's generation_id fixes