Add a "fork" of musl as //fusl.

fusl/src/process/posix_spawnattr_sched.c

+#include <spawn.h>
+#include <sched.h>
+#include <errno.h>
+
+int posix_spawnattr_getschedparam(const posix_spawnattr_t *restrict attr,
+        struct sched_param *restrict schedparam)
+{
+        return ENOSYS;
+}
+
+int posix_spawnattr_setschedparam(posix_spawnattr_t *restrict attr,
+        const struct sched_param *restrict schedparam)
+{
+        return ENOSYS;
+}
+
+int posix_spawnattr_getschedpolicy(const posix_spawnattr_t *restrict attr, int *restrict policy)
+{
+        return ENOSYS;
+}
+
+int posix_spawnattr_setschedpolicy(posix_spawnattr_t *attr, int policy)
+{
+        return ENOSYS;
+}