[m-rev.] for review: restrict search for a bootstrap compiler in configure.ac

[Fabrice Nicol]

I am posting this as a follow-up to my user list post (here: 
http://lists.mercurylang.org/archives/users/2022-January/009425.html).

I propose restricting the search for a bootstrap compiler to DEV-flavour 
versions.

For ROTDs and releases, this search is avoided by enforcing 
BOOTSTRAP_MC="" and the corresponding configure message is not displayed.

This will avoid user issues when, for some reason, a user has an older 
"broken" (though possibly quite fresh) compiler in PATH and does not 
realise it should be removed for the configure script to move on. (This 
is not an automatic reflex with newcomers, and understandably so).

It was tested in the following configurations:

- mercury 22.0 beta release, with "polluted" and clean paths: configure 
always succeeds.

- mercury git clone in the same two cases: configure fails without a 
bootstrap compiler in PATH; succeeds otherwise.

This is more of a "user-friendly" modification than a bug fix. At least 
it will avoid some unnecessary feedback.

Fabrice
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