Log Message:
Update waf to 1.8.15:

NEW IN WAF 1.8.15
-----------------
* Enabled multiple commands in run_str/rule
* Fixed conf.check(header_name=) so that it does not link test by default
* Added a workaround for non-standard python installations (Python bug 7352) - #1636
* Enabled conf.check_cfg to use conf.env.env as os environment if provided
* Enabled relative paths in --top and --out
* Enabled gzip compression for Waf file creation with Jython
* Added a workaround for building projects using IronPython
* Added a workarounds for pkg-config usage within msys2 and for inconsistent \ 
Python 2 paths