Upgrade to waf 1.8.4.

This version adds support for `clang` and `clang++` compilers on
Linux and OSX so that they can be used directly and not just when
the `gcc` and `g++` commands actually map to Clang.

Tested on Linux, Solaris, AIX, and Windows.

Signed-off-by: Andrew Paprocki <andrew@ishiboo.com>