hwloc-ps could work on cygwin

[bgoglin]

It would have to use hwloc_pid_from_number() first.
It's currently not built on any windows flavor.

See also #330

For testing, note that hwloc-bind doesn't work (#151) hence you have to run a program that autobinds itself, or to use the task manager for binding an existing program.

[bgoglin]

lstopo --ps works on cygwin since de0b368, we likely just need to reenable building of hwloc-ps on cygwin to fix this issue.