make hello take an optional argument

src/main.c

@@ -1,6 +1,13 @@
 #include <stdio.h>
 
-int main(void) {
-	printf("Hello, world!\n");
+int main(int argc, char **argv) {
+	const char *s = "world";
+
+	if (argc > 1) {
+		s = (const char*) argv[1];
+	}
+
+	printf("Hello, %s!\n", s);
+
 	return 0;
 }