[m-dev.] Mercury.options files

[Zoltan Somogyi]

During a recent change, I found that the formatting of our
Mercury.options files is inconsistent, in that some use spaces
for indentation, others use tabs, and some use a mix of both (ugh).

It is a minor thing, but I would like consistency. I don't much care whether
we pick spaces or tabs, but I think we should pick one and stick to it,
using vim mode lines to enforce our choice to either expand or not expand
tabs.

On the side of tabs, we could use ft=make for syntax highlighting,
since Mercury.options files function as parts of Makefiles.

On the side of spaces, Mercury.options files are not actually bodily
included in Makefiles/Mmakefiles, and we use spaces for such files.

Any votes one way or another, or in favor of the current chaos?

Zoltan.