[Patch, Fortran] (F2015) Permit ERROR STOP in PURE procedures

[Tobias Burnus]

It often happens that one wants to debug PURE procedures but as I/O is 
not permitted, this can be difficult. F2008's IMPURE ELEMENTAL helps a 
bit not completely.

Fortran 2015 adds another aid – not I/O which is intrinsically impure – 
but at least ERROR STOP is now permitted.

Attached is this belate Christmas present for Damian and the second 
F2015 for GCC 5. (The first was "IMPLICIT NONE (external)".)

Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias
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