[musl] _GNU_SOURCE and _LARGEFILE_SOURCE

[Brian Cain <quic_bcain@quicinc.com>]

In the "WHATSNEW" text, there's an item from 0.9.2 that states "- make 
_GNU_SOURCE imply _LARGEFILE64_SOURCE".  Is that intended to be the case 
generally?  I ask because in 25e6fee2 (remove LFS64 programming 
interfaces (macro-only) from _GNU_SOURCE, 2022-09-27) it stated 
"portable software should be prepared for them not to exist" and "the 
intent is that this be a very short-term measure and that the macros be 
removed entirely in the next release cycle."

This comes up because in the QEMU project, there's a linux multiarch 
test case that uses readdir64 and it does define _GNU_SOURCE but does 
not define _LARGEFILE64_SOURCE and as such the cross compiler complains 
that there's no declaration of readdir64 before the call site.  I 
suppose that the test case would be more portable if it defined 
_LARGEFILE64_SOURCE.  But I'd also be happy to send a patch to musl that 
could have _GNU_SOURCE imply _LARGEFILE64_SOURCE (again?) if that's 
desired.  But - I gather that defining the macros is not what we want.  
Instead of macros I should add declarations for readdir64() and its 
LFS64 friends, but only when _LARGEFILE64_SOURCE is defined?

-Brian