https://bugs.gentoo.org/715780

musl #ifdef:s most of the defines in sched.h with _GNU_SOURCE
https://elixir.bootlin.com/musl/latest/source/include/sched.h#L13.

---

diff --git a/src/benchmark.c b/src/benchmark.c
index eb96b2a..18fc2d2 100644
--- a/src/benchmark.c
+++ b/src/benchmark.c
@@ -40,6 +40,7 @@
 #endif
 
 #ifdef HAVE_SCHED_SETAFFINITY
+#define _GNU_SOURCE
 #define __USE_GNU
 #include <sched.h>
 #endif
-- 
2.35.1