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).

Regtested on x86_64-linux, OK to commit?
FX


:ADDPATCH fortran: