Bump minimum MPFR version to 3.1.0

Bump minimum MPFR version to 3.1.0

Bump the minimum MPFR version to 3.1.0, released 2011-10-03. With this
requirement one can still build GCC with the operating system provided
MPFR on old but still supported operating systems like SLES 12 (MPFR
3.1.2) or RHEL/CentOS 7.x (MPFR 3.1.1).

This allows removing some code in the Fortran frontend, as well as
fixing PR 91828.

ChangeLog:

2019-11-11  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91828
	* configure.ac: Bump minimum MPFR to 3.1.0, recommended to 3.1.6+.
	* configure: Regenerated.

gcc/ChangeLog:

2019-11-11  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91828
	* doc/install.texi: Document that the minimum MPFR version is
	3.1.0.

gcc/fortran/ChangeLog:

2019-11-11  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91828
	* simplify.c (gfc_simplify_fraction): Remove fallback path for
	MPFR < 3.1.0.

From-SVN: r278058