[m-dev.] configure option(s) to disable different back-ends

[Fergus Henderson]

Hi,

For the GCC back-end interface, I need to add a configure option
for the Mercury compiler to enable/disable the GCC back-end.
This option is needed because building with the GCC back-end interface enabled
requires having a copy of GCC to link with, and we don't want to
require everyone to do that.

However, shouldn't there be a similar option to configure to disable
the Aditi back-end?  `--disable-aditi-back-end'?

(For consistency, the GCC back-end configure option would be called
`--enable-gcc-back-end'; note that for now, at least, it will be
disabled by default.)

You can already disable the Aditi back-end by setting a variable in
Mmake.params, but I think it would be nicer to provide this as a
configure option too.

In the long run, it might even be desirable to have options to
configure to selectively disable the other back-ends.  However,
currently that would require a lot of work.  In particular many
places in the other back-ends rely on parts of the LLDS back-end.
We would need to do a lot of work to improve the modularity of the
compiler before disabling the LLDS back-end would be feasible.

Note that we already have options to disable building the libraries
for various grades.  But those options don't stop the relevant parts
of the compiler needed to compile code in those grades from being
linked into the mercury_compile executable.

Comments?

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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