PATCH: Allow gcc/gthr* to be compiled by C++ as well as C

[Loren James Rittle]

>> Why it is type (void*) instead of int is unknown to me.

> Perhaps it should be

>   static int __gthread_active_ptr = !! &pthread_create;

> then?

No disagreement from me.  However, changing the type of
__gthread_active_ptr is an ABI change instead of a mere bug fix to
allow C++ compilation of the header...

I am wiling to do more clean up on those files since I understand
them, but I didn't want to radically change anything.

Regards,
Loren