[build2] Custom Configuration Variables for the Config Module

[Boris Kolpackov]

Markus Pawellek <markus.pawellek at mailbox.org> writes:

> Right now, I am little bit embarassed.
> Until now, I have not read the NEWS file or the currently existing
> issues on GitHub. My questions are already answered and a solution
> seems to be already implemented. So I am very sorry that I have
> bothered you with this message.

No worries, that's quite a happy coincidence that the functionality
you need was just implemented ;-).

While the documentation for this feature will be available in the
usual place after the release next week, in the meantime here it
is from the staged version:

https://build2.org/stage/build2/doc/build2-build-system-manual.xhtml#proj-config

I believe what's currently supported should allow you to achieve
everything you've described. But let me know if you find anything
missing.