Changes

Fix st_stubs.c compilation with mingw-w64 13.0.0

In mingw-w64 13.0.0, time.h now causes pthread_compat.h to be included
which as a side-effect sets up the macros for declspec(dllimport). Since
caml/osdeps.h uses time.h, this means that the macro changes designed to
ensure the API functions are properly decorated doesn't get applied, and
and RELOC_REL32 errors abound.

The fix for now is to ensure that the macros are setup to control
pthread_compat.h at the very beginning of the file.