Waf 1.9
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* Fix the vala detection
* Better consistency between check_cfg and check_cc variables
* Let more context commands depend on the configuration
* Rework qt5
* Use relative paths in apply_incpaths (and absolute ones when paths cross drives)
* Provide a more efficient ConfigSet implementation
* Ensure _cache.py are valid python files
* One task.idx per task node
* Other issues listed on https://github.com/waf-project/waf/issues