[RFA] AIX thread support

[Benjamin Kosnik]

David this patch seems incorrect. 

If you are saying that --enable-threads=aix implies that the posix threads layer is going to be 
used, then there is no reason to make a new thread header. Just reuse the 
posix (pthreads) one.

also please don't put threads stuff in _CHECK_OS, put it in _ENABLE_THREADS

>     case "$target_thread_file" in  
>       no | none | single)
>         THREADH=threads-no.h
> +       ;;
> +     aix)
> +       THREADH=threads-aix.h
>         ;;
>       posix | pthreads)
>         THREADH=threads-posix.h

should be:

>     case "$target_thread_file" in  
>       no | none | single)
>         THREADH=threads-no.h
>         ;;
>       posix | pthreads | aix)
>         THREADH=threads-posix.h

-benjamin