[Bug fortran/92463] New: Cleanups due to minimum MPFR version bump to 3.1.0

[jb at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92463

            Bug ID: 92463
           Summary: Cleanups due to minimum MPFR version bump to 3.1.0
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jb at gcc dot gnu.org
  Target Milestone: ---

As part of resolving PR 91828, the minimum MPFR version required to build GCC
was bumped to 3.1.0 from the previous 2.4.0. 

This enables a bunch of cleanups, mostly in the Fortran frontend (hence
assigning this bug to the fortran component).

From https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01544.html :

> For example, you could justify a move to requiring MPFR 3.0.0 or later 
with cleanups to use MPFR_RND* instead of the older GMP_RND*, and 
similarly mpfr_rnd_t instead of the older mp_rnd_t and likewise mpfr_exp_t 
and mpfr_prec_t in fortran/.  You could justify a move to requiring MPC 
1.0.0 (or 1.0.2) by optimizing clog10 using mpc_log10.  I don't know what 
if any newer GMP interfaces would be beneficial in GCC.  And as always in 
such cases, it's a good idea to look at e.g. how widespread the newer 
versions are in GNU/Linux distributions, which indicates how many people 
might be affected by an increase in the version requirement.