Comment author: @dbuenzli

Actually they are build correctly with the -thread. The problem seems more to be on ocamlfind (or a wrong invocation of ocamlfind):

ocamlbuild -classic-display -tag thread -use-ocamlfind tsdl.cma
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlopt -I /Users/dbuenzli/.opam/4.00.1/lib/ocaml/ocamlbuild unix.cmxa /Users/dbuenzli/.opam/4.00.1/lib/ocaml/ocamlbuild/ocamlbuildlib.cmxa myocamlbuild.ml /Users/dbuenzli/.opam/4.00.1/lib/ocaml/ocamlbuild/ocamlbuild.cmx -o myocamlbuild
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamldep -modules support/consts.ml > support/consts.ml.depends
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -c -annot -thread -I support -I test -I src -o support/consts.cmo support/consts.ml
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -ccopt -DHAS_SDL2 -ccopt -D_THREAD_SAFE -ccopt -I/usr/local/include/SDL2 -ccopt -I/usr/X11R6/include -c support/consts_stub.c
mv consts_stub.o support/consts_stub.o
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -linkpkg -custom -thread support/consts.cmo support/consts_stub.o -o support/consts.byte
support/consts.byte src/tsdl_consts.ml
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamldep -modules src/tsdl_consts.ml > src/tsdl_consts.ml.depends
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamldep -package ctypes -modules src/tsdl.mli > src/tsdl.mli.depends
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -c -annot -thread -package ctypes -I src -I test -I support -o src/tsdl.cmi src/tsdl.mli
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamldep -package ctypes -modules src/tsdl.ml > src/tsdl.ml.depends
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -c -annot -thread -I src -I test -I support -o src/tsdl_consts.cmo src/tsdl_consts.ml
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -c -annot -thread -package ctypes -I src -I test -I support -o src/tsdl.cmo src/tsdl.ml
/Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -a -cclib -L/usr/local/lib -cclib -lSDL2 -package ctypes -package ctypes.foreign src/tsdl_consts.cmo src/tsdl.cmo -o src/tsdl.cma

• /Users/dbuenzli/.opam/4.00.1/bin/ocamlfind ocamlc -a -cclib -L/usr/local/lib -cclib -lSDL2 -package ctypes -package ctypes.foreign src/tsdl_consts.cmo src/tsdl.cmo -o src/tsdl.cma
  ocamlfind: Error from package `threads': Missing -thread or -vmthread switch
  Command exited with code 2.

Comment author: @gasche

This should be fixed since #6058. Thanks for the feedback anyway!

PS: the problem indeed comes from the fact that ocamlfind's "error" mechanism is not flexible enough to express the difference between linking into a final program (where -thread is needed) and linking into an archive (when -thread is not needed), while OCamlbuild did this fine-grained distinction. The solution is to dumb down OCamlbuild and pass -thread each time.

In the other bugtracker item, william proposed a workaround that may interest you before the release: -lflags -thread.