Use extern instead of caml/threads.h

src-c/native/stubs.c

@@ -37,7 +37,17 @@
     return Val_unit;                                                         \
   }
 #else
-#include <caml/threads.h>
+/* XXX(dinosaure): even if they are not defined (only defined by
+ * [caml/threads.h]), they exists without [threads.cmxa]. For compatibility
+ * reason, we keep a protection when we compile for Solo5/ocaml-freestanding
+ * but for the rest, these functions should be available in any cases.
+ *
+ * In some cases (Solo5 or Esperanto), [caml/threads.h] is not available but
+ * these functions still exist!
+ */
+
+extern void caml_enter_blocking_section (void);
+extern void caml_leave_blocking_section (void);
 
 #define __define_ba_update(name)                                             \
   CAMLprim value                                                             \