Unfortunately, it looks like the Cephes library of mathematical functions linked from this article is released with a poorly specified license, and contains code that is copyrighted by multiple organizations: http://www.netlib.org/cephes/readme
[Edit: I overlooked the fact that, being free under GPL, the GSL implementations do not suit the author's intent. (See /jwmerrill's comment below). Leaving the comment anyway just for reference, but my original point is moot.]
This. The author should have at least mentioned the ubiquituous and reasonably well-maintained GNU Scientific Library (GSL) [1], which already implements all the functions listed in the article [2,3,4,5], among many, many other things [6].
The gsl is a great resource, but I don't think it suits the author's purpose. He is talking about adding new functions to the C standard library, for anyone to use in commercial or non-commercial software. It isn't practical to stick GPL code into C standard libraries that are meant to be used broadly in commercial as well as free software.
GSL's design doc (https://www.gnu.org/software/gsl/design/gsl-design.html) explicitly names many libraries, including CEPHES, that its creators consider useful but with drawbacks, such as non-free licensing, that motivated GSL's creation.
I'm not especially familiar with GSL except for a few functions I've used on occasion, and not with CEPHES at all, so I can't speak to other pros or cons.
Presumably if the "C" committee decided to do this, implementers would be free to implement this by various means, whether it be a clean-room version, or by adopting existing code. In the latter case, they could contact the author(s) of Cephes and try to arrange more suitable licensing if needed.
Admittedly it adds a little extra friction to the process, but this doesn't seem like an insurmountable problem off hand.
