[libstdc++] Expose Airy functions.

[Jonathan Wakely]

On 16/11/17 13:52 -0500, Ed Smith-Rowland wrote:
>We've had these Airy functions almost to the surface for a while now.
>
>Let's have them as extensions.
>
>This is undergoing testing now.
>
>OK if testing passes?

The code is fine (the new functions are only defined when #ifndef
__STRICT_ANSI__ like the hypergeometric extensions).

Some of the new tests are actually HTML files though!

Please remember to CC the gcc-patches list with the updated patch.
It's OK for trunk assuming that it passes all tests, with the HTML
files fixed.

Also, this reminds me to ask about the __isnan helper in
<tr1/special_function_util.h>, can we get rid of it? It looks to me
like we always have a usable std::isnan, no?