Since VCD has now to go through the T tracer, we enforce the consistency
of data found in common/utils/LOG/vcd_signal_dumper.[ch] with data
found in common/utils/T/T_messages.txt.

We might get completely rid of common/utils/LOG/vcd_signal_dumper.[ch] at some
point. For the moment, let's keep it.

This commit adds the program common/utils/T/check_vcd.c and necessary
modifications to enforce its use at compilation time.

If you modify common/utils/LOG/vcd_signal_dumper.[ch] but do not update
common/utils/T/T_messages.txt an error will pop up when you compile the
software. You have to keep both modules synchronized.