diff: fix for purity + intermodule opt bug

[Peter Schachte]

> Peter Schachte, you might want to have a glance at this.
>
> compiler/hlds_out.m:
> 	Make sure that we write out purity annotations (the previous
> 	failure to do this caused errors in the generated `.opt' files
> 	when compiling with intermodule optimization).
> 
> compiler/purity.m:
> compiler/mercury_to_mercury.m:
> 	Move the definition of `write_purity_prefix' from
> 	mercury_to_mercury.m to purity.m, and make it exported,
> 	for use by hlds_out.m.

This looks fine.

-Peter Schachte			| Yesterday upon the stair I met a man who
pets at cs.mu.OZ.AU		| wasn't there. He wasn't there again today
http://www.cs.mu.oz.au/~pets/	| -- I think he's from the CIA. 
PGP key available on request	|