Add link flags for external dependencies when using ctypes.

Required when the external lib is not statically linked, in which case
linker needs to be aware at runtime what libs to call. This is the case
for

- dune exec <file>.bc
- dune utop
- mdx tests

That is add to ocamlmklib, link flags such as -l<libname> -L<libdir>
from ctypes if info is available.

If dependent library are not installed in expected locations, env
variable such as LD_LIBRARY_PATH must be extended when required program
is run, depending on os. This may be fixed by adding relocatable paths to
link flags.

Signed-off-by: Frej Soya <frej.soya@gmail.com>

src/dune_rules/lib_rules.ml

@@ -332,7 +332,12 @@ let build_stubs lib ~cctx ~dir ~expander ~requires ~dir_contents ~vlib_stubs_o_f
       | _ -> Action_builder.return []
     in
     let c_library_flags =
-      Expander.expand_and_eval_set expander lib.c_library_flags ~standard
+      let open Action_builder.O in
+      let* c_lib = Expander.expand_and_eval_set expander lib.c_library_flags ~standard in
+      let+ ctypes_lib =
+        Ctypes_rules.ctypes_cclib_flags sctx ~expander ~buildable:lib.buildable
+      in
+      List.append c_lib ctypes_lib
     in
     let lib_o_files_for_all_modes = Mode.Map.Multi.for_all_modes o_files in
     let for_all_modes = List.rev_append vlib_stubs_o_files lib_o_files_for_all_modes in

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/dune

@@ -0,0 +1,4 @@
+(executable
+ (name example)
+ (modes byte)
+ (libraries examplelib))

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/dune-project

@@ -0,0 +1,3 @@
+(lang dune 3.10)
+
+(using ctypes 0.3)

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/example.ml

@@ -0,0 +1,2 @@
+let () =
+  Printf.printf "%d\n" (Examplelib.C.Functions.add2 2)

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/run.t

@@ -0,0 +1,23 @@
+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
+
+  $  PKG_CONFIG_PATH="$LIBEX/pkgconfig" PKG_CONFIG_ARGN="--define-prefix" DYLD_LIBRARY_PATH="$DYLD_LIBRARY_PATH:$LIBEX" LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$LIBEX" CAML_LD_LIBRARY_PATH="$CAML_LD_LIBRARY_PATH:$PWD/_build/default/stubgen" dune exec ./example.bc
+  4
+
+Utop works with ctypes pkg-config external library.
+
+  $   DYLD_LIBRARY_PATH="$DYLD_LIBRARY_PATH:$LIBEX" LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$LIBEX"  PKG_CONFIG_PATH="$LIBEX/pkgconfig" PKG_CONFIG_ARGN="--define-prefix" dune utop --display=short ./ -- example.ml
+    pkg-config stubgen/.pkg-config/libexample.cflags
+    pkg-config stubgen/.pkg-config/libexample.libs
+        ocamlc .utop/.utop.eobjs/byte/dune__exe__Utop.{cmi,cmo,cmt}
+        ocamlc .utop/utop.bc
+  4

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/stubgen/dune

@@ -0,0 +1,14 @@
+(library
+  (name examplelib)
+  (flags (:standard -w -9-27))
+  (ctypes
+    (external_library_name libexample)
+    (build_flags_resolver pkg_config)
+    (headers (include "example.h"))
+    (type_description
+      (instance Types)
+      (functor Type_description))
+    (function_description
+      (instance Functions)
+      (functor Function_description))
+    (generated_entry_point C)))

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/stubgen/function_description.ml

@@ -0,0 +1,8 @@
+open Ctypes
+
+module Types = Types_generated
+
+module Functions (F : Ctypes.FOREIGN) = struct
+  open F
+  let add2 = foreign "example_add2" (int @-> returning int)
+end

test/blackbox-tests/test-cases/ctypes/bytecode-stubs-external-lib.t/stubgen/type_description.ml

@@ -0,0 +1,3 @@
+module Types (F : Ctypes.TYPE) = struct
+
+end

test/blackbox-tests/test-cases/ctypes/dune

@@ -27,6 +27,7 @@
 
 (cram
  (applies_to
+  bytecode-stubs-external-lib
   lib-pkg_config
   lib-pkg_config-multiple-fd
   lib-external-name-need-mangling