support for GNU configure syntax

[Cynede]

It will be useful for unix systems, some package managers and external tools that is passing / maybe looking for GNU configure syntax options.

One of related bugs: #5138

Also that could be useful in future if project will really install man pages / other stuff using those variables.

[sanxiyn]

If we were to do this (I am not sure about that) I think we should eliminate the old syntax instead of having both old syntax and GNU syntax.

[pnkfelix]

cc me

[brson]

I agree that if we do this it should replace the existing options. --build, --host and --target seem like reasonble replacements for our --build-triple, --host-triples and --target-triples options. (It looks like this patch doesn't actually use these flags for anything, just accepts them?)

I'm not sure the role of the other options besides --mandir which we could support in an obvious way.

Making this change will require some automation and documentation changes too.

I guess I'm inclined to move in this direction for the sake of compatibility. Probably the way to do it is:

• Convert to using --build, --host, and --target, leaving the current flags for temporary compatibility.
• Wire up the others that make sense so that they do something.
• Leave the ones that don't have any meaning for us as no-ops?
• Update the various documentation to use the new flags.
• Merge.
• Update automation.
• Remove old flags.

Anybody else have opinions?

[steveklabnik]

Compatibility is good.

[Cynede]

@brson I'm agreed, that will be much better. Will you take this task or will wait for me? I can try to do it ~next week.

[brson]

@Heather you can do it.

[Cynede]

@brson

• where are those flags are using?
• How can I handle compatibility with older syntax there?
• noopt will fail if package manager will run configure --mandir=/usr/...

[alexcrichton]

Closing this temporarily to see if it un-sticks bors.

[alexcrichton]

It appears that this has ended up confusing bors, I think it's because you opened the pull request from the branch heather, but now your commits are coming from master (in your repo) instead. Not sure how github is figuring everything out, but would you mind re-opening in a new pull-request to un-stick bors? Thanks!

[Cynede]

Yes, sorry, I usually use to break git repositories with rebasing and branch was broken so I recreated it on master on fork.