[patch, fortran] Warn about integer**(negative integer)

[FX]

> 2017-01-22  Thomas Koenig  <tkoenig@netcologne.de>
> 
>        * arith.c (arith_power):  If simplifying integer power expression
>        to zero, warn if -Winteger-division is given.
> 
> 2017-01-22  Thomas Koenig  <tkoenig@netcologne.de>
> 
>        * gfortran.dg/integer_exponentiation_7.f90:  New test.

The idea seems reasonable, but I am not sure about the wording (“truncated” in particular). “Integer exponentiation truncated to constant 0”? Why not “Negative integer exponent has result value zero”?

Other than that, OK to commit.

FX