[gfortran,patch] Add ISNAN intrinsic

[FX Coudert]

:REVIEWMAIL:

> This patch adds an ISNAN intrinsic as a GNU extension, since others  
> compilers have it (ifort, g95, sunf95, sgi) and some code relies on  
> it (http://www.libatoms.org/). The patch is simple, comes with a  
> testcase and documentation written by Tobias Burnus (slightly  
> modified by me).

Received green light by Steve Kargl on IRC and committed as rev. 127224.

FX