subst vars for arches without native compilers?

[Eric Cooper <ecc@cmu.edu>]

While updating approx's control file, I realized it's broken on
architectures that require ocamlrun because they lack a native compiler.

I looked at Sylvain's packaging of Unison, which would have the same
issue. He creates an "interpreter:Depends" variable in the rules file,
set to ocaml-base-NNNN or empty, depending on the architecture.  Is
this a reasonable model to follow?  What's the policy for naming
substitution variables?

-- 
Eric Cooper             e c c @ c m u . e d u