Log Message:
waf: updated to 2.0.21

NEW IN WAF 2.0.21
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* Set the default --msvc_version from VSCMD_VER if present
* Force unit-test reruns on ut_str, ut_cmd or ut_path changes
* Describe Qt5's library detection
* Introduce conf.env.ASMDEFINES_ST to enable assembly-specific define flags
* Update extras/xcode6 to Python3
* Enable parameter "always" in extras/doxygen
* Fix extras/c_dumbpreproc as it was previously broken
* Fix extras/gccdeps and extras/msvcdeps on header renaming
* Improve extras/msvcdeps debug outputs and flags
* Add add MCST Elbrus CPU detection in c config
* Add minio object copies to extras/wafcache