Build an example library as a DLL and set up the environment so that it looks
like a system/distro library that can be probed with pkg-config and dynamically
loaded.

  $ LIBEX=$(realpath "$PWD/../libexample")

ocamlrun + requires CAML_LD_LIBRARY_PATH such that dlopen system call can find
dllexamplelib_stub.so

Explictly set {DYLD,LD}_LIBRARY_PATH at runtime for this testcase, otherwise
dlopen cannot find libexample, after loading dllexamplelib_stub.so

  $ export PKG_CONFIG_PATH="$LIBEX/pkgconfig"
  > export PKG_CONFIG_ARGN="--define-prefix"
  > export DYLD_LIBRARY_PATH="${DYLD_LIBRARY_PATH:-}:$LIBEX"
  > export LD_LIBRARY_PATH="${LD_LIBRARY_PATH:-}:$LIBEX"
  > export CAML_LD_LIBRARY_PATH="${CAML_LD_LIBRARY_PATH:-}:$PWD/_build/default/stubgen"
  > dune exec ./example.bc
  4

Utop works with ctypes pkg-config external library.

  $ dune utop ./ -- example.ml
  4

