Include <string.h> for the strcmp function.  This avoids an implicit
function declaration and build failures with future compilers that no
longer support them by default.

diff -ur man-1.6g.orig/src/makemsg.c man-1.6g/src/makemsg.c
--- man-1.6g.orig/src/makemsg.c	2006-08-02 20:11:44.000000000 +0200
+++ man-1.6g/src/makemsg.c	2023-02-07 19:14:51.354523820 +0100
@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <string.h>
 #ifdef __QNX__
 #include <unix.h>
 #endif