Fix 32-bit compilation on MS-Windows

Fix 32-bit compilation on MS-Windows

* src/jit.c (CONSTANT): Avoid compilation warning on 32-bit
systems about casting a pointer to an integer of a different
size.
(compile) [HAVE__SETJMP]: Use _setjmp instead of setjmp, when
the latter is a macro.  Fixes a compilation error on system
that use _setjmp.
(Fjit_disassemble_to_string): Rearrange declaration to avoid
compilation warnings on systems that don't have open_memstream.