Change preprocessor OS tests to __linux__
"linux" without leading underscores is deprecated and less reliable.
diff --git a/threads.c b/threads.c
index 8c57263..e52f263 100644
--- a/threads.c
+++ b/threads.c
@@ -48,7 +48,7 @@
static int libxml_is_threaded = -1;
#if defined(__GNUC__) && defined(__GLIBC__)
-#ifdef linux
+#ifdef __linux__
#if (__GNUC__ == 3 && __GNUC_MINOR__ >= 3) || (__GNUC__ > 3)
#pragma weak pthread_once
#pragma weak pthread_getspecific
@@ -68,7 +68,7 @@
#pragma weak pthread_key_delete
#pragma weak pthread_cond_signal
#endif
-#endif /* linux */
+#endif /* __linux__ */
#endif /* defined(__GNUC__) && defined(__GLIBC__) */
#endif /* HAVE_PTHREAD_H */