You've described an excellent case for it being so

Yes, I think you're right. I'll research it a bit more and then run it by p5p and see what happens.

There may be a view that it's up to the make utility to set this correctly - and that I really ought to be using different make utilities for the different compilers.
(There's one way to find out whether such a view prevails :-)

Anyway ... thanks for the encouragement !!

Cheers,
Rob