Make datapath compile with Xen kernel.

Make datapath compile with Xen kernel.

The Xen kernel is based on 2.6.18 but backports many features from later
kernels.  It is not always possible, therefore, to detect whether we need
to use compatibility code based on LINUX_VERSION_CODE.  This commit fixes
the problem by using configure-time tests to check for the need for the
compatibility code.

Build-tested on Linux 2.6.15 through 2.6.28 with the default configuration
(except that some kernels needed preemption turned off) and with Xen
kernel 2.6.18-92.1.10.el5.xs5.0.0.394.644.

Fixes bug #548.