Log Message:
mpfr: update to 4.2.1.

Changes from version 4.2.0 to version 4.2.1:
- Bug fixes (see <https://www.mpfr.org/mpfr-4.2.0/#fixed> and/or the
  ChangeLog file).
- Improved MPFR manual.
- Configure tests: replaced the test of the link with GMP, in order to
  avoid the use of a function without a prototype (Autoconf issue), as
  this is obsolescent in ISO C. The new test should be more robust.