build: work with older RHEL 5 kernel

build: work with older RHEL 5 kernel

We already skip out on building the LXC under RHEL 5, because the
kernel is too old (commits 4c18acf, 2dee896); but commit 9612e4b
moved some LXC-only code into common files, resulting in this
build failure:

util/virfile.c: In function 'virFileLoopDeviceAssociate':
util/virfile.c:580: error: 'LO_FLAGS_AUTOCLEAR' undeclared (first use in this function)

Unfortunately, the kernel folks only made it an enum, rather than
also a #define, so we have to modify configure.ac to record when
it is usable.

* configure.ac (with_lxc): Mark when LO_FLAGS_AUTOCLEAR was found.
* src/util/virfile.c (virFileLoopDeviceAssociate): Avoid
compilation when kernel is too old.