[Bug fortran/91828] gcc/fortran/check.c requires mpfr_set_z_2exp from MPFR 3.0.0, unavailable in mpfr-2.4.2

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91828

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Requiring 3.1.x is OK by now.  SLES 12 has 3.1.2, only SLES 11 and older cannot
fulfil this requirement.