Georgi pointed out that some overly-pedantic C libraries require you to include both "string.h" and "strings.h". (Most don't.)
diff --git a/toys.h b/toys.h
index aa43ae7..34cf035 100644
--- a/toys.h
+++ b/toys.h
@@ -28,6 +28,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
 #include <sys/mount.h>