cmake maintenance

• allow meaningful ENABLE_XHOST=OFF for PGI #422, though perhaps it'd be more consistent to warn in words that ENABLE_XHOST only adds arch targeting flags
• sorry, I had already made the edits when I realized #444 already had a proposed solution with !604 . I thought I'd post anyways, then you can choose as you like.
  • fills in version variables. (unless now would be a good time to drop configure.ac?)
  • REMOVED prepares alias targets
  • SKIPPED namespacing options unaddressed. I'm indifferent on project-unique options, but I have real reservations on namespacing standard cmake variables. Settling dual input like BUILD_SHARED_LIBS and Libxc_BUILD_SHARED_LIBS in unwieldy cmake seems harder than using the standard var and letting it propagate through the projects or, if fine control wanted, building each project separately.