Use -c not -s, that is used for a fixed size in master now

swiftmpistepsim.c

@@ -447,7 +447,7 @@ int main(int argc, char *argv[]) {
   /* Handle the command-line, we expect a mpiuse data file to read and various
    * options. */
   int opt;
-  while ((opt = getopt(argc, argv, "vfds:")) != -1) {
+  while ((opt = getopt(argc, argv, "vfdc:")) != -1) {
     switch (opt) {
       case 'd':
         datacheck = 1;
@@ -458,7 +458,7 @@ int main(int argc, char *argv[]) {
       case 'v':
         verbose = 1;
         break;
-      case 's':
+      case 'c':
         messagescale = atof(optarg);
         break;
       default: