posixmodule.c doesn't detect xattr support on Linux with non-glibc (e.g. musl)

Bug report

posixmodule.c detects xattr support by checking for __GLIBC__ (and some other conditions) at

Line 277 in 86ebd5c

    
           #if defined(HAVE_SYS_XATTR_H) && defined(__GLIBC__) && !defined(__FreeBSD_kernel__) && !defined(__GNU__)

. This incorrectly excludes the musl libc,which supports xattr functionality.

On a musl system, it's easy to reproduce with:

>>> import os
>>> os.listxattr 
Traceback (most recent call last): 
	File "", line 1, in
AttributeError: module 'os' has no attribute 'listxattr'

CPython versions tested on: 3.10.10/3.11.2/3.12.0_alpha5
Operating system and architecture: Gentoo Linux, amd64, musl libc (not glibc)
Downstream report in Gentoo: https://bugs.gentoo.org/894130
Report in a Python-using project: AttributeError: module 'os' has no attribute 'listxattr' GNS3/gns3-gui#1392