build: convert protocol variables to dicts

[emersion]

I'd prefer to have a dict instead (e.g. protocols_code.get(name) would return the file for the generated code for instance).

That's actually a pretty good idea. Unfortunately meson (0.52.1) can't use a non string literal for a dictionary key:

protocol_code = {}
foreach proto, path : protocols
    # ...
    protocol_code += { proto: code } # This is invalid
endforeach

And due to the way dictionaries work (i.e. they're pure) and some dictionary-of-dictionaries type stuff, I can't really think of a way to work around this (e.g. putting it in the protocols dictionary).

But this will be fixed in 0.53.0: https://mesonbuild.com/Syntax.html#dictionaries
It has been left as set_variable/get_variable currently, but changed to use an array of protocol strings in the wayland backend.
I think this should be revisited when the next meson version is out and we eventually target it for whatever reason.

Originally posted by @ascent12 in #1931

[emersion]

When we bump to 0.53, we'll also be able to use summary instead of message.