recognise system's mpfr version via mpfr_free_pool presence, generate SAGE_MPFR_PREFIX to be used with --with-mpfr= options.

the branch to be merged as a part of #27822

Depends on #27641

CC: @embray @kiwifb

Component: build: configure

Keywords: spkg-configure mpfr

Author: Dima Pasechnik, Erik Bray

Branch: 0329fd9

Reviewer: Erik Bray, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/27258