build: honor user supplied Xen tree before checking for xenstore headers
build: search user supplied directory for shared libraries
Adapt to new domctl interface

The upcoming Xen 4.5 has extended the CPU affinity interface. Choose
the right interface via versioning. The cpumap_hard is the new name for
cpumap in pre-4.5, so behavior is preserved.
Merge pull request #33 from liuw/misc

Misc fixes