sys/signal.h is not the POSIX appropriate place for signal.h, which mes libc
attempts to conform to.

diff -r -U3 src/m4.c src/m4.c
--- src/m4.c	1994-11-02 03:14:28.000000000 +0000
+++ src/m4.c	2021-01-15 00:19:21.947654461 +0000
@@ -19,7 +19,7 @@
 #include "m4.h"
 
 #include <getopt.h>
-#include <sys/signal.h>
+#include <signal.h>
 
 static void usage _((int));