Allow command checkers to specify environment for starting processes

[doublep]

This simple change adds possibility for command-based checkers to specify environment (i.e. environment variables) in which their processes are started. This is completely optional and not used by any existing checker. I plan to eventually use this functionality in flycheck-eldev. In Eldev it is possible to switch Emacs binary you use, but not as a command line option (command line is parsed by Elisp code, so it's a bit too late), only by modifying an environment variable.

Verified by make check specs unit. Also verified that it actually affects process environment e.g. by using this locally:

:environment (lambda (checker environment) (cons "ELDEV_EMACS=lol" environment))

After this Eldev couldn't launch, as expected.

[CLAassistant]

All committers have signed the CLA.

[cpitclaudel]

Thanks, the patch is clean and simple.

However, I'm a bit unsure whether we actually need this, at least just yet, since no other checker makes use of this. It has come up a few times, but never really became needed.

(As to picking an emacs version, what I've done in esh is to use a minimal wrapper that calls an Emacs subprocess according the relevant environment variable, I think)

[doublep]

I'm a bit unsure whether we actually need this, at least just yet, since no other checker makes use of this.

Of course, no checker can make use of something that doesn't exist.

It has come up a few times, but never really became needed.

Aha. But I guess each time a workaround like

what I've done in esh is to use a minimal wrapper that calls an Emacs subprocess

was used instead of a "clean and simple" feature.

[cpitclaudel]

Of course, no checker can make use of something that doesn't exist.

This is not what I meant. What I mean is that we've never have a case yet where this was really needed, I think.

Aha. But I guess each time a workaround like … was used instead of a "clean and simple" feature.

No, not really, at least not in my memory (esh isn't a Flycheck checker)

Besides,

what I've done in esh is to use a minimal wrapper that calls an Emacs subprocess

Isn't that what you do too? Otherwise, how do you use the environment variable?

[doublep]

Isn't that what you do too? Otherwise, how do you use the environment variable?

Shell script: https://github.com/doublep/eldev/blob/master/bin/eldev

What I want to do is to use the oldest available Emacs binary that is still good enough for the current project (most projects declare which Emacs version they require). This would allow to spot incompatibilities with older Emacsen using Flycheck, e.g. usage of too new functions.