Enable static linking

[Wenzel]

Use pkg-config crate to search for xenstore.pc and retrieve the Library configuration in order to statically link libxenstore, when the static feature has been specified.

A couple of bugs and gotchas here

• the .statik() doesn't have any effect because it's conditioned by the static library being outside of /usr/lib. (see Static linking is too restrictive rust-lang/pkg-config-rs#102). On top of that, the author couldn't remember why this restriction was put in place, and there hasn't been any new release yet that would remove this code
• The Library struct returned by pkg-config doesn't expose the Required.private field, which is necessary to specify that we need to link with xentoolcore as well, since xenstore depends on it. So we need to specify this manually. It may break in the future if Xenstore adds or remove any of its dependencies unfortunately