Get OCaml headers from the ocaml directory

Before, most headers were duplicated in the include directory (and some
important onces were missing).
Install a pkg-config file for mirage-xen

This allows programs compiling C stubs to find the mirage-xen header
files. It also includes the dependencies on openlibm and Mini-OS.

The Libs: entry is not currently used, but in future the mirage tool
may prefer to use this rather than specifying the linker options
manually.
Sync CHANGES for 2.1.0