[Nix-dev] (Optionally) Build same package multiple ways on Hydra

[Michael Raskin]

>Consider the emacs-23 package as an example, it has an optional ‘gtk’
>argument we can set and is checked for null.
>
>So in a config one might have something like
>
>      (emacs23.override { gtk = gtk3; })
>
>Unfortunately this means that one always has to build emacs themselves.
>It would be great if package maintainers could provide some default
>arguments with which Hydra will build.

>This is only an idea at this point but what do you think? Yes, there
>would be extra things to build by Hydra, can it afford it? I don't
>imagine the implementation would be extremely difficult but I don't have
>experience here.

If the option combination deserves its own Hydra job, it deserves
a top-level attribute in all-packages.nix (which is a one-line addition)
for ease of use with «nix-env -i». 

If there are specific packages you think were missed by such treatment,
please provide a list/pull-request/link to pastebin… I personally don't
think we need a new mechanism here, but evaluating what was missed over
time is probably a good idea.