mpfr: upgrade 4.2.0 -> 4.2.1

mpfr: upgrade 4.2.0 -> 4.2.1

Changelog:
===========
- 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.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 7d0699e176ad1b50c26c5981c0ee976615994729)
Signed-off-by: Steve Sakoman <steve@sakoman.com>