[patch,fortran] PRs 19363 and 19691, Cygwin failure in formatted write of Inf and NaN

[Paul Thomas]

libgfortran.h has already got a test in it for missing isfinite(x).  The 
fastest fix would be to extend this to the case of the broken isfinite 
in Cygwin.  I know you do not like it Steven but it does put all the 
nastiness in one place!  Maybe if I submit a patch to do that and then 
undertake to shift the whole lot to configure, when it gets back to the 
top of the todo pile?

Paul T