waf: update to 2.0.12.

NEW IN WAF 2.0.12
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* Fix broken inheritance task trees #2194

NEW IN WAF 2.0.11
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* Do not raise an exception on check_cfg/mandatory=False/-vv #2193
* Post past task generators in lazy sub-folder builds #2191
* Disable warnings on versioned library installation
* Fix cpplint concurrent execution problems

(wiz)