Please stage together with mpc

- Update to mpfr 4.0.0
* The "dinde aux marrons" release.
* MPFR now depends on GMP 5.0+ instead of 4.1+.
* API change:
Applications that call GMP's mp_set_memory_functions function to change
the allocators must first call the new function mpfr_mp_memory_cleanup
in all threads where MPFR is potentially used; this new function is
currently equivalent to mpfr_free_cache.
The reason is that the way memory allocation is done by MPFR has changed
(again), so that the current GMP allocators are used (since for some
applications, the old allocators may become invalid).
Note: Freeing the caches like this might have a performance impact on some
particular applications; if this is an issue, this could be handled for a
future MPFR version.
* Mini-gmp support via the --enable-mini-gmp configure option (experimental).
* The minimum precision MPFR_PREC_MIN is now 1, with rounding defined as
in the errata of IEEE 754-2008 and in the following IEEE 754 revision
(ties rounded away from zero).
* Shared caches for multithreaded applications.
New function mpfr_free_cache2.
* Partial support of MPFR_RNDF (faithful rounding).
* New functions: mpfr_fpif_export and mpfr_fpif_import to export and import
numbers in a floating-point interchange format, independent both on the
number of bits per word and on the endianness.
* New function mpfr_fmodquo to return the low bits of the quotient
corresponding to mpfr_fmod.
* New functions mpfr_flags_clear, mpfr_flags_set, mpfr_flags_test,
mpfr_flags_save and mpfr_flags_restore to operate on groups of flags.
* New functions mpfr_set_float128 and mpfr_get_float128 to convert from/to
the __float128 type (requires --enable-float128 and compiler support).