gh-101857: Allow xattr detection on musl libc (#101858)

Previously, we checked exclusively for `__GLIBC__` (AND'd with some other
conditions). Checking for `__linux__` instead should be fine.

This fixes using e.g. `os.listxattr()` on systems using musl libc.

Bug: https://bugs.gentoo.org/894130

Co-authored-by: Gregory P. Smith <[email protected]>

Author: thesamesam

Misc/NEWS.d/next/Core and Builtins/2023-02-12-22-40-22.gh-issue-101857._bribG.rst

@@ -0,0 +1 @@
+Fix xattr support detection on Linux systems by widening the check to linux, not just glibc. This fixes support for musl.

Modules/posixmodule.c

@@ -274,8 +274,9 @@ corresponding Unix manual entries for more information on calls.");
 #  undef HAVE_SCHED_SETAFFINITY
 #endif
 
-#if defined(HAVE_SYS_XATTR_H) && defined(__GLIBC__) && !defined(__FreeBSD_kernel__) && !defined(__GNU__)
+#if defined(HAVE_SYS_XATTR_H) && defined(__linux__) && !defined(__FreeBSD_kernel__) && !defined(__GNU__)
 #  define USE_XATTRS
+#  include <linux/limits.h>  // Needed for XATTR_SIZE_MAX on musl libc.
 #endif
 
 #ifdef USE_XATTRS