bug#25987: 25.2; support gcc fixit notes

[Dmitry Gutov <dgutov@yandex.ru>]

On 08.03.2017 21:28, Eli Zaretskii wrote:

> Quick fixes work by presenting a kind of tooltip.  We should probably
> have something similar, but it cannot be the only UI, to support TTY
> and GUI users who don't like the mouse.

Tooltip or not, that's a visualization concern.

I think a better first question would be where this feature would fit 
among the existing Emacs tools.

If we look at Eclipse and friends, we can observe that (almost?) every 
warning or error that the IDE shows to the user comes with a set of 
actions, suggesting possible fixes, improvements, etc.

Thinking along these lines, the closest things we have are Flymake and 
Flyspell. The third-party Flycheck is even better is several respects 
(e.g. it can show a listing of all problems in the file buffer), but 
sadly we won't be able to use it, since the primary author is sternly 
against copyright assignment.

But it'd be good to teach Flymake about fixits, and when we can't use 
tooltips, use a text-based prompts a la Flyspell.

Bonus points for teaching Flymake to use multiple checkers in one file, 
and making Flyspell one of them.

I guess the first difficulty is how to reconcile project-global 
compilation logs with buffer-local warnings and errors.