Support negation of requirements

[Flamefire]

If you want to build something on Windows only my current solution was: <build>no <target-os>windows:<build>yes

However this causes conflicts when any other rule (e.g. C++11 requirements) adds a <build>no too

Hence it would be great if negation like <target-os>!windows:<build>no would be supported

The current solution involves a rather clunky conditional and a function

[mloskot]

#594 is related as per https://lists.boost.org/Archives/boost/2020/05/248931.php

[Flamefire]

If I read this correctly this already exists as !<target-os>windows:<build>no. Is this correct?

[Kojoley]

Yes, but there seems to be another quirk that prevents to use negation on the first statement. I opened a PR with the error message reporting fix #611, and you are welcome to fix the bug itself.

[stale[bot]]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2 This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.