restore attribute((const)) to pthread_self and errno location decls
revert commit a603a75a72bb469c6be4963ed1b55fabe675fe15.
as a result of commit 1c84c99913bf1cd47b866ed31e665848a0da84a2 this is
now safe, assuming an interpretation of the somewhat-underspecified
attribute((const)) consistent with real-world usage.
diff --git a/include/pthread.h b/include/pthread.h
index bba9587..e238321 100644
--- a/include/pthread.h
+++ b/include/pthread.h
@@ -79,6 +79,9 @@
_Noreturn void pthread_exit(void *);
int pthread_join(pthread_t, void **);
+#ifdef __GNUC__
+__attribute__((const))
+#endif
pthread_t pthread_self(void);
int pthread_equal(pthread_t, pthread_t);