make: fix GCC warnings

*** 337,347 ****
           * Only prepend the /usr/bin/nice command to the original command
           * if the nice priority, nice_prio, is NOT zero (0).
           * Nice priorities can be a positive or a negative number.
           */
          if (nice_prio != 0) {
!                 argv[argv_index++] = NOCATGETS("nice");
                  (void) sprintf(nice_prio_buf, NOCATGETS("-%d"), nice_prio);
                  argv[argv_index++] = strdup(nice_prio_buf);
          }
          argv[argv_index++] = shellname;
  #if defined(linux)
--- 337,347 ----
           * Only prepend the /usr/bin/nice command to the original command
           * if the nice priority, nice_prio, is NOT zero (0).
           * Nice priorities can be a positive or a negative number.
           */
          if (nice_prio != 0) {
!                 argv[argv_index++] = (char *)NOCATGETS("nice");
                  (void) sprintf(nice_prio_buf, NOCATGETS("-%d"), nice_prio);
                  argv[argv_index++] = strdup(nice_prio_buf);
          }
          argv[argv_index++] = shellname;
  #if defined(linux)