Log Message:
waf: update to 2.0.20.

NEW IN WAF 2.0.20
-----------------
* Detect -flto and -fno-lto in parse_config #2281
* Improve custom option processing #2280
* Enable Clang on cygwin #2279
* Make distclean target 'no_lock_in_top/run' modifiers compatible with env vars #2271
* Update irix compiler detection
* Exclude ConfigSet from altering ConfigurationContext.run_build caches #2273
* Add gas support in extras/gccdeps.py #2278
* Improve compatibility with custom commands in \ 
extras/clang_compilation_database #2274