Don't undef <unit>_TRACE, instead add comment how to enable it
In units that support detailed reference tracing via ISC_REFCOUNT macros, we were doing:
/* Define to 1 for detailed reference tracing */
#undef <unit>_TRACEThis would prevent using -D_TRACE=1 in the CFLAGS.
Convert the above mentioned snippet with just a comment how to enable the detailed reference tracing:
/* Add -D<unit>_TRACE=1 to CFLAGS for detailed reference tracing */