[m-dev.] turning on --halt-at-warn-if-possible

[Zoltan Somogyi]

I have a diff that add this new option to the .mgnuc_opts file in each
directory. I have just committed a diff that fixes the last thing
that causes gcc warnings during a hlc.gc bootcheck, so it is good to go,
but I want to wait until (a) everyone has installed a compiler with the
9d46463 change I committed yesterday, and (b) there is an rotd
with that change. I would like to add this option to .mgnuc_opts files
on thursday. If this is too early for you, please speak up.

Zoltan.