Use Linux functions on any OS based on GNU libc

GNU libc (glibc) provides the prototypes for all the xattr functions.

Hence, extend the Linux support to anything based on GNU libc. In case
of failures, most likely the underlying xattr functions will need to
be fixed, rather than pyxattr itself.

xattr.c

@@ -23,7 +23,7 @@
 
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
-#if defined(__APPLE__) || defined(__linux__)
+#if defined(__APPLE__) || defined(__linux__) || defined(__GLIBC__)
 #include <sys/xattr.h>
 #endif
 #include <stdio.h>
@@ -224,7 +224,7 @@ static inline int _fremovexattr(int filedes, const char *name) {
     return fremovexattr(filedes, name, 0);
 }
 
-#elif defined(__linux__)
+#elif defined(__linux__) || defined(__GLIBC__)
 #define _listxattr(path, list, size) listxattr(path, list, size)
 #define _llistxattr(path, list, size) llistxattr(path, list, size)
 #define _flistxattr(fd, list, size) flistxattr(fd, list, size)