Filters: If somebody doesn't like _Thread_local, don't fail for now, just be a little slower.

When the parallel execution comes into place, we'll likely enforce this
C11 feature. It's much simpler and also faster than pthread_[sg]etspecific().

lib/birdlib.h

@@ -73,6 +73,12 @@ static inline int u64_cmp(u64 i1, u64 i2)
 #define UNUSED __attribute__((unused))
 #define PACKED __attribute__((packed))
 
+#ifdef HAVE_THREAD_LOCAL
+#define THREAD_LOCAL _Thread_local
+#else
+#define THREAD_LOCAL
+#endif
+
 /* Microsecond time */
 
 typedef s64 btime;