Thanks for the patch!  I probably won't get to this properly until after 
2.6 final, but it won't get lost.  It seems like there's pretty good 
support for adding these functions.

By the way, there's already a setup in Python 2.6 (ad 3.0) for substitutes 
for missing math functions:  take a look at the file Python/pymath.c, 
which provides inverse hyperbolic functions for those platforms that need 
them, as well as the bits in configure.in that check for these functions.  
This is the really the right place for the tgamma/lgamma/erf/erfc code.

So a full patch for this should touch at least Python/pymath.c, 
Modules/mathmodule.c, configure.in, Lib/test/test_math.py, and 
Doc/Library/math.rst.  If you have time to fill in any of these pieces, it 
would be greatly appreciated.