test: ctypes and relative include paths

This adds a test that shows that under `(using ctypes 0.2)`, local
headers need to be added in two different locations because rules are
executed from different places. See ocaml#5325.

Signed-off-by: Etienne Millon <me@emillon.org>

test/blackbox-tests/test-cases/ctypes/directories.t

@@ -0,0 +1,76 @@
+This test characterizes how the ctypes support deals with directories, in
+particular when relative paths are passed as `-I`.
+
+With `(using ctypes 0.2)`, some commands are executed in workspace root and
+others are executed in the directory where `(ctypes)` is found, so it is
+necessary to pass relative dirs twice. (this prevents vendorability, since this
+requires knowing the path from the workspace root to the current directory).
+
+To test that, we create a binding to `BUFSIZ` and `fopen`, both in the C
+library. These are provided by `stdio.h`, but instead of using this header,
+we're proxying it through a `lib.h` which should be in the include path for
+compilation to work.
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.4)
+  > (using ctypes 0.2)
+  > EOF
+
+  $ mkdir lib
+
+  $ cat > lib/dune << EOF
+  > (executable
+  >  (name e)
+  >  (flags :standard -w -27)
+  >  (ctypes
+  >   (build_flags_resolver
+  >    (vendored (c_flags :standard (:include extra_flags.sexp))))
+  >   (external_library_name l)
+  >   (headers (include lib.h))
+  >   (deps lib.h)
+  >   (type_description
+  >    (functor f)
+  >    (instance types))
+  >   (function_description
+  >    (functor f)
+  >    (instance functions))
+  >   (generated_entry_point entry)))
+  > EOF
+
+  $ echo "(-I lib -I .)" > lib/extra_flags.sexp
+
+  $ cat > lib/lib.h << EOF
+  > #include <stdio.h>
+  > EOF
+
+  $ cat > lib/e.ml << EOF
+  > let () = Printf.printf "%d\n" Entry.Types.bufsiz
+  > let (_:char Ctypes.ptr -> char Ctypes.ptr -> Entry.Functions.file) = Entry.Functions.fopen
+  > EOF
+
+  $ cat > lib/f.ml << EOF
+  > module Types(F:Ctypes.TYPE) = struct
+  >   open F
+  >   let bufsiz = constant "BUFSIZ" int
+  > end
+  > 
+  > module Functions(F:Ctypes.FOREIGN) = struct
+  >   open Ctypes
+  >   open F
+  >   type file = unit ptr
+  >   let file : file typ = ptr void
+  >   let fopen = foreign "fopen" (ptr char @-> ptr char @-> returning file)
+  > end
+  > EOF
+
+  $ dune build
+
+We ensure that just `-I lib` or `-I .` are not enough on their own.
+
+  $ echo "(-I lib)" > lib/extra_flags.sexp
+  $ dune build > /dev/null 2>&1
+  [1]
+
+  $ echo "(-I .)" > lib/extra_flags.sexp
+  $ dune build > /dev/null 2>&1
+  [1]