adaption to g++-7

[UnixDino]

vathpela: "gcc 7 introduces detection of fall-through behavior in switch/case
statements, and will warn if -Wimplicit-fallthrough is present and there
is no comment stating that the fall-through is intentional."

proposed for
/mpfr_bin_ieee754_flavor_io_impl.hpp ...
at end of case p1788::io::no_bounds_special_representation:
(line 354):
/* falls through - necessary for g++-7 */

after this correction the translation with g++-7 and all tests are working