Log Message:
math/py-pandas: fix aggregations.pyx so that this builds on NetBSD.

On NetBSD, signbit and sqrt cannot be imported from "libc", because
even though <math.h> is included, Python.h later includes
<cmath> which in turn #undef's all the relevant definitions
from <math.h>, forcing the use of the std:: variants of those
functions.  So ... re-do how we import signbit() and sqrt().