[m-dev.] Drop SICStus & NU-Prolog support?

[Warwick Harvey]

Fergus writes:
> Therefore I propose that we drop the existing support for SICStus
> and NU-Prolog.
> 
> Comments?

This will mean I can get rid of my local hack to stop it attempting to build 
a SICStus version (I got fed up with overnight builds failing due to changes 
made in the latest version of SICStus).  I never used it anyway.  So I'm in 
favour.  :-)

Warwick