fix config.txt

src/maketools/update-config-txt.sh

@@ -103,6 +103,7 @@ echo "$version"
 echo
 echo "# command to lauch mpi processes"
 echo "# syntax: mpiexec command"
+echo "# empty string means that mpiexec was not chosen at configure time"
 echo "mpiexec $mpiexec"
 echo
 echo "# list of 'has' options"