make it compile under MSN vc. Unfortunately it has a weird crash at boot :(

- Lots of indenting changes
- VC++ is strict with variadic macros
- VC++ does not accept unistd.h
- new interface for walltime
- VC++ does not seem to have support for integer overflow.
- VC++ defines YENV_REG?
- no access flags, x permissions ignored.
- new FindGMP supporting MPIR
- make horus optional (c++ is hard).

os/sig.c

@@ -363,7 +363,7 @@ Yap_MathException__( USES_REGS1 )
       return  EVALUATION_ERROR_UNDEFINED;
     }
   }
-#elif _WIN32 && FALSE
+#elif _WIN32
   unsigned int raised;
   int err;