mpfr: Update to version 4.2.1

mpfr: Update to version 4.2.1

- Update from version 4.2.0 to 4.2.1
- Update of rootfile
- Changelog
    4.2.1
	patches 1 to 12 from 4.2.0 have been built in to 4.2.1
	Other bugs fixed in the 4.2 branch for the MPFR 4.2.1 release:
	    The + and space flags were ignored on NaN and Inf. While this was loosely
	     documented as such (without an explicit mention of these flags), the MPFR
	     manual also says that the flags have the same meaning as for the standard
	     printf function. So this was contradictory and regarded as a bug. Behaving
	     like the ISO C standard should give less surprise, and this is probably
	     what is expected (better for alignment purpose). See discussion (only for
	     NaN and the + flag at that time).
	     Corresponding changeset in the 4.2 branch: 3761bee3c.
	    Huge negative exponents can trigger integer overflows in mpfr_strtofr,
	     meaning undefined behavior. Two bugs have been identified: 1, 2. In
	     practice, the consequences may be incorrect results. But for the first bug,
	     it has been seen that a GCC optimization makes it invisible. There are
	     other issues with the code for huge exponents, but it is not clear whether
	     the problematic cases can occur in the context of mpfr_strtofr; such
	     potential bugs are not fixed yet.
	     Corresponding changesets in the 4.2 branch: 261d3852b (tests), 06e7b6bc1
	      (bug fixes).

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>