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).
OK for trunk. Can you also make a note in https://gcc.gnu.org/gcc-10/changes.html ? Regards Thomas