V3 PATCH: Allow an OS to override _G_USING_THUNKS

[Benjamin Kosnik]

> N.B. -- I changed the tests of _G_USING_THUNKS in libioP.h to use
> `#if' rather than `#ifdef'.  That's so that the _G_config.h file can
> do:
> 
>   #ifndef _G_USING_THUNKS
>   # define _G_USING_THUNKS	1
>   #endif /* _G_USING_THUNKS */
> 
> rather than something like:
> 
>   #ifndef _G_DONT_USE_THUNKS
>   #define  _G_USING_THUNKS
>   #endif 
> 
> If that's problematic for some reason, please let me know, and I'll be
> happy to do something else instead.

should be fine

-benjamin