configure: don't build the configure script from the Makefile

This triggers a build loop under some circumstances. Instead, make a
"configure" shell script which builds the configure program and runs it.

Signed-off-by: David Scott <dave.scott@citrix.com>
xenlight: detect a compatible version of xenlight directly, rather than inferring based on Xen version

The Xen version in some places is not identifiably either xen-4.4 or xen-4.5.

Signed-off-by: David Scott <dave.scott@citrix.com>
configure: a certain distribution has backported xen-4.5's generation id to xen-4.4

This means we can nolonger detect xen-4.4 by looking for the generation id.

Instead, assume that a new enough xenlight implies xen-4.4 (or above)

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: invoke configure

Signed-off-by: David Scott <dave.scott@citrix.com>
configure: it's bash, not sh

Signed-off-by: David Scott <dave.scott@citrix.com>
configure: add a --disable-xenguest argument

For distributions for which we cannot build any sensible xenguest, disable
it entirely and use libxl.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #60 from djs55/simplify-configure

Be more specific with xenlight detection