Fix for PR 15235 libgfortran doesn't build on Solaris

[Rainer Orth]

Tobias Schlüter writes:

> Rainer, what are the ways to check for NaN on Solaris? From what I see
> on the SunOS box I have access to, isnan is in math.h, and is therefore
> available on all systems. Is that right?

It is in Solaris 8 (the oldest version I have access to), and the Solaris
2.4 man pages on docs.sun.com include it as well.

	Rainer