Optional threading support

[yallop]

Functorize the foreign package to provide a thread-safe version and a version that doesn't use threads. The META file is set up so that ocamlfind will link against the right version according to the flags used to build the client library.

Without -thread:

$ ocamlfind ocamlc -verbose -linkpkg -package ctypes.foreign tst.cmo -o tst
Effective set of compiler predicates: pkg_unix,pkg_bigarray,pkg_ctypes,\
    pkg_ctypes.foreign-base,pkg_ctypes.foreign,autolink,byte
+ ocamlc.opt -verbose -o tst -I /home/jeremy/.opam/4.01.0/lib/ctypes \
      /home/jeremy/.opam/4.01.0/lib/ocaml/unix.cma \
      /home/jeremy/.opam/4.01.0/lib/ocaml/bigarray.cma \
      /home/jeremy/.opam/4.01.0/lib/ctypes/ctypes.cma \
      /home/jeremy/.opam/4.01.0/lib/ctypes/ctypes-foreign-base.cma \
      /home/jeremy/.opam/4.01.0/lib/ctypes/ctypes-foreign-unthreaded.cma \
      tst.cmo

With -thread:

$ ocamlfind ocamlc -verbose -thread -linkpkg -package ctypes.foreign tst.cmo -o tst
Effective set of compiler predicates: pkg_unix,pkg_threads.posix,pkg_threads,pkg_bigarray,pkg_ctypes,\
    pkg_ctypes.foreign-base,pkg_ctypes.foreign,autolink,mt,mt_posix,byte
+ ocamlc.opt -verbose -o tst -thread -I /home/jeremy/.opam/4.01.0/lib/ctypes \
      /home/jeremy/.opam/4.01.0/lib/ocaml/unix.cma \
      /home/jeremy/.opam/4.01.0/lib/ocaml/threads/threads.cma \
      /home/jeremy/.opam/4.01.0/lib/ocaml/bigarray.cma \
      /home/jeremy/.opam/4.01.0/lib/ctypes/ctypes.cma \
      /home/jeremy/.opam/4.01.0/lib/ctypes/ctypes-foreign-base.cma \
      /home/jeremy/.opam/4.01.0/lib/ctypes/ctypes-foreign-threaded.cma \
      tst.cmo