use getprogname if we have, otherwise punt, remove roken dependency

lib/sl/sl.c

@@ -53,8 +53,11 @@ mandoc_template(SL_cmd *cmds,
     t = time(NULL);
     strftime(timestr, sizeof(timestr), "%b %d, %Y", localtime(&t));
     printf(".Dd %s\n", timestr);
-    p = strrchr(getprogname(), '/');
-    if(p) p++; else p = getprogname();
+#ifdef HAVE_GETPROGNAME
+    p = getprogname();
+#else
+    p = "unknown-application";
+#endif
     strncpy(cmd, p, sizeof(cmd));
     cmd[sizeof(cmd)-1] = '\0';
     strupr(cmd);