strtonum isn't available on <Darwin-11

however, it is in the header files on systems like 10.15 with an
availability clause, so don't define strtonum static such that it
matches the prototype from stdlib.h

diff --git a/netcat.c b/netcat.c
index a0fb51b..bbb5dd1 100644
--- a/netcat.c
+++ b/netcat.c
@@ -240,6 +243,43 @@ static int connect_with_timeout(int fd, const struct sockaddr *sa,
 
 static void quit();
 
+static char* strtonumerrs[] = {
+	"too large",
+	"too small",
+	"invalid"
+};
+
+long long
+strtonum(
+		const char *nptr,
+		long long minval,
+		long long maxval,
+		const char **errstr)
+{
+	long long val;
+
+	while (*nptr != '\0' && isspace(*nptr))
+		nptr++;
+	if (*nptr == '\0') {
+		if (errstr != NULL)
+			*errstr = strtonumerrs[2];
+		return 0;
+	}
+	val = atoll(nptr);
+	if (val < minval) {
+		if (errstr != NULL)
+			*errstr = strtonumerrs[1];
+		return 0;
+	}
+	if (val > maxval) {
+		if (errstr != NULL)
+			*errstr = strtonumerrs[0];
+		return 0;
+	}
+	*errstr = NULL;
+	return val;
+}
+
 int
 main(int argc, char *argv[])
 {