[m-dev.] Proposal: Improve specifying Mercury install paths

[Julien Fischer]

On Sun, 5 Apr 2020, Julien Fischer wrote:

>>  Running with a --bindir arg bypasses $prefix and $exec_prefix as we
>>  would expect:
>>
>>     $ ./configure --bindir=/usr/bin
>>
>>     prefix = ...
>>     exec_prefix=${prefix}
>>     MERCURY_COMPILER=${MERCURY_COMPILER-"/usr/bin/mercury_compile"}
>
> That should be fine; I'll try your patch out.  Mercury and autoconf are
> something of an imperfect fit anyway.  So far as Mercury is concerned
> the supported configuration options for setting installation directories
> will now be:
>
>     --prefix
>     --bindir
>     + whatever ones are require for setting documentation directories.
>
> (The others, such as --exec-prefix, don't really make sense for Mercury.)
>
> We'll need to document all this somewhere, probably in .INSTALL.in.
> (Unfortunately, it doesn't seem possible to change the usage message
> for the configure script.)

I've committed the patch.  There are a few circumstances where --bindir
is not currently applied, in the batch files we generate for Windows and
also in the wrapper scripts used to invoke an installed compiler when it
is built in the C# or Java grades -- I will look into fixing those case
separately.

Julien.