With a non strict version of numpy (e.g. 1.8.1~rc1 as in debian/sid currently), the version check fails with a ValueError because it calls int() with the 'rc1' string.
One solution for this would be to use distutils's version comparison framework. In waftools/python_cfg.py check_numpy_version(), something like:
from distutils.version import LooseVersion result = LooseVersion(npyver) >= LooseVersion(minver_str)