configure: add a verbose mode

[mathstuf]

Required for Fedora infrastructure so that the commands used to build
ninja are logged.

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Port of #265 to the new configure.py script.

[nico]

Thanks!

It's a bit strange that this only has an effect in bootstrap mode, but I couldn't think of anything better. (It could be --bootstrap and --bootstrap=verbose, but that's strange too.)

[mathstuf]

What else would it output? "wrote build.ninja"? Maybe a list of files it thinks are important and re2c detection?

[nico]

I don't think it should print more. It just feels a bit strange that there's a flag (that shows up in -help and all) that has no visible effect unless you pass another flag too.

[mathstuf]

Ah, I see now. I see it as "the only place where there is currently code to be verbose about is during bootstrap". There's certainly nothing stopping it from having effects other than bootstrap (but is the place it makes the most sense). I guess the other place that it would make sense is in has_re2c where you might want to print the version found. The rest is mainly just writing out the initial build.ninja file.