Update waf to head of master branch

Update waf to commit 20a1ed61468cc793c0a79de83ea1b7005de0ce88, which is
currently the head of master in the official waf git repo.

Fixes 3 submitted issues:
https://code.google.com/p/waf/issues/detail?id=1511
https://code.google.com/p/waf/issues/detail?id=1512
https://code.google.com/p/waf/issues/detail?id=1513