configure.ac: fix the maximum number of workers when cpu is disabled

 - we need a number big enough for the mpi master slave workers, not
   sure what is the correct way of knowing how many workers we need in
   advance, as it wil depend on the number of mpi processes

configure.ac

@@ -2410,10 +2410,11 @@ if test x$enable_simgrid != xyes; then
         nmaxmpidev=1
     fi
 fi
-nmaxworkers=`expr 16 \* \( \( \( $nmaxmpidev \* $maxcpus \) + $nmaxcudadev + $nmaxopencldev + 15 \) / 16 \) `
-if test $nmaxworkers = 0
+if test $maxcpus == 0
 then
-	nmaxworkers=16
+	nmaxworkers=`expr 16 \* \( \( \( $nmaxmpidev \* 64 \) + $nmaxcudadev + $nmaxopencldev + 15 \) / 16 \) `
+else
+	nmaxworkers=`expr 16 \* \( \( \( $nmaxmpidev \* $maxcpus \) + $nmaxcudadev + $nmaxopencldev + 15 \) / 16 \) `
 fi
 AC_MSG_CHECKING(Maximum number of workers)
 AC_MSG_RESULT($nmaxworkers)