Implement stack arrays even for unknown sizes

[Dominique Dhumieres]

> The resulting speed up for nf.f90 is rather remarkable.  What specific
> feature of the fortran leads to a 30=>15s ?

I think it is the automatic array in the subroutine trisolve. Note that the 
speedup is rather 27->19s and may be darwin specific (slow malloc).

Note also that -fstack-arrays prevents some optimizations on
fatigue: 4.7->7s. This may be related to pr45810.

Dominique