switch to mpfr 4.2.0

[thvnx]

See: https://www.mpfr.org/mpfr-4.2.0/ (should fix CI for macos)

Changes from versions 4.1.* to version 4.2.0

• 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).