Log Message:
mpfr: update to 4.2.0.

Changes from versions 4.1.* to version 4.2.0:
- The "fondue savoyarde" release.
- Binary compatible with MPFR 4.0.* and 4.1.*, though some minor changes in
  the behavior of the formatted output functions may be visible, regarded
  as underspecified behavior or bug fixes (see below).
- New functions mpfr_cosu, mpfr_sinu, mpfr_tanu, mpfr_acosu, mpfr_asinu,
  mpfr_atanu and mpfr_atan2u.
- New functions mpfr_cospi, mpfr_sinpi, mpfr_tanpi, mpfr_acospi, mpfr_asinpi,
  mpfr_atanpi and mpfr_atan2pi.
- New functions mpfr_log2p1, mpfr_log10p1, mpfr_exp2m1, mpfr_exp10m1 and
  mpfr_compound_si.
- New functions mpfr_fmod_ui, mpfr_powr, mpfr_pown, mpfr_pow_uj, mpfr_pow_sj
  and mpfr_rootn_si (mpfr_pown is actually a macro defined as an alias for
  mpfr_pow_sj).
- Bug fixes.
  In particular, for the formatted output functions (mpfr_printf, etc.),
  the case where the precision consists only of a period has been fixed
  to be like ".0" as specified in the ISO C standard, and the manual has
  been corrected and clarified.
  The macros of the custom interface have also been fixed: they now behave
  like functions (except a minor limitation for mpfr_custom_init_set).