[PATCH threads.xs] no system scope on VMS

On all but the latest VMS versions, the pthread_attr_setscope function 
is unavailable.  Even where it is available it is a no-op since only 
process scope is supported.  The patch here simply excludes the call.

• [PATCH threads.xs] no system scope on VMS by Craig A. Berry
• Re: [PATCH threads.xs] no system scope on VMS by Jeremy Begg
• Re: [PATCH threads.xs] no system scope on VMS by Craig A. Berry