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

[Thomas Koenig]

Hello world,

a reacent thread on c.l.f showed another common error - expecting
10**(-3) to expect something different from zero.  The attached
patch warns about this result if -Winteger-division is active.

Why -Winteger-division?  Two reasons:  First, 10**(-3) ist just
a shorthand for integer division.  Second, I did not want to add
yet another warning flag.

Regression-tested.  OK for trunk?

	Thomas

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.
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