fix(ctypes): 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>

doc/changes/8784.md

@@ -0,0 +1 @@
+- Add link flags to to ocamlmklib for ctypes stubs (#8784, @frejsoya)

src/dune_rules/lib_rules.ml

@@ -328,7 +328,14 @@ 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
+      and+ ctypes_lib =
+        (* CR rgrinberg: Should we add these flags to :standard? to make
+           it possible for users to remove these *)
+        Ctypes_rules.ctypes_cclib_flags sctx ~expander ~buildable:lib.buildable
+      in
+      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/run.t

@@ -11,10 +11,7 @@ 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
-  Fatal error: cannot load shared library dllexamplelib_stubs
-  Reason: $TESTCASE_ROOT/_build/default/stubgen/dllexamplelib_stubs.so: undefined symbol: example_add2
-  Aborted
-  [134]
+  4
 
 Utop works with ctypes pkg-config external library.
 
@@ -23,7 +20,4 @@ Utop works with ctypes pkg-config external library.
     pkg-config stubgen/.pkg-config/libexample.libs
         ocamlc .utop/.utop.eobjs/byte/dune__exe__Utop.{cmi,cmo,cmt}
         ocamlc .utop/utop.bc
-  Fatal error: cannot load shared library dllexamplelib_stubs
-  Reason: $TESTCASE_ROOT/_build/default/stubgen/dllexamplelib_stubs.so: undefined symbol: example_add2
-  Aborted
-  [134]
+  4