[build2] Recommended way to override build2 configuration CLI arguments

[Boris Kolpackov]

Matthew Krupcale <mkrupcale at matthewkrupcale.com> writes:

> On Thu, Mar 7, 2019 at 9:20 AM Boris Kolpackov <boris at codesynthesis.com> wrote:
> >
> > But can add support for straight assignment in the meantime, if
> > that would be helpful.
> 
> Yes, I think that would be helpful.

I went ahead and implemented the whole thing, which means now not only
can we do:
    
$ b config.cxx.coptions=-O3 config.cxx.coptions=-O0
    
But also:

$ b config.cxx.coptions=-O3 config.cxx.coptions+=-g

We are aiming to make a release early next week so if you could give
this a try before then, that would be great.

The latest staged toolchain is probably the easiest way to test this
since the changes are somewhat substantial and are spread over several
commits. Alternatively, you can try to apply them to 0.9.0:

https://git.build2.org/cgit/build2/commit/?id=355534c22ab547bceae189e11ce36fa0dea4e743
https://git.build2.org/cgit/build2/commit/?id=d1b3ad7b302d037c8154bab9c4810d499c0bf1e4
https://git.build2.org/cgit/build2/commit/?id=e20a351013745e8d6c3a0a99bd40c172ed0ae8be