long double-related changes in gnulib

[barracuda156]

@iains Gnulib has added float-related stuff recently, which in its current form breaks gettext-runtime (at least its latest release, 0.26), which in turn means a lot of stuff will be broken. Looks like sources are not aware of macOS existence – all macros are Linux- or AIX-specific.

These are relevant files:
https://github.com/coreutils/gnulib/blob/f4038dcb346fccb58d910e2f0a62c0f45022d2a8/lib/float.c
https://github.com/coreutils/gnulib/blob/f4038dcb346fccb58d910e2f0a62c0f45022d2a8/lib/float.in.h

These commits also refers to gcc15 changes in relation to C23:
coreutils/gnulib@65ed9d3
coreutils/gnulib@1ce8849

What should we do to fix this? Both for cases of the latest gcc being used and Xcode gcc (which so far worked to build gettext subports).