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

[joseph at codesourcery dot com]

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

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
My view is as stated in 
<https://gcc.gnu.org/ml/gcc-patches/2016-09/msg01602.html> and 
<https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01544.html>:

* Update the minimum to at least 3.0, possibly later if there are 
relevant features in later versions it would be convenient to use 
unconditionally.

* Clean up the uses of obsolescent MPFR interfaces such as those I listed 
in <https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01544.html>.  Many of 
those are just renamings, but it's still better for GCC to use the main 
interfaces people will know about from reading the MPFR manual rather than 
ones that, if documented at all, are documented as deprecated names.