Setting the STD_CDEFINES environment variable before running configure can
be used to enable certain compile-time options that are not explicitly
defined in configure.

Some of these settings are:

Setting                   Description
                          Overwrite memory with tag values when allocating
-DISC_MEM_DEFAULTFILL=1   or freeing it; this impairs performance but
                          makes debugging of memory problems easier.
                          Don't track memory allocations by file and line
-DISC_MEM_TRACKLINES=0    number; this improves performance but makes
                          debugging more difficult.
-DISC_FACILITY=LOG_LOCAL0 Change the default syslog facility for named
-DNS_CLIENT_DROPPORT=0    Disable dropping queries from particular
                          well-known ports:
-DCHECK_SIBLING=0         Don't check sibling glue in named-checkzone
-DCHECK_LOCAL=0           Don't check out-of-zone addresses in
                          named-checkzone
-DNS_RUN_PID_DIR=0        Create default PID files in ${localstatedir}/run
                          rather than ${localstatedir}/run/named/
                          Disable the use of inline functions to implement
-DISC_BUFFER_USEINLINE=0  the isc_buffer API: this reduces performance but
                          may be useful when debugging
-DISC_HEAP_CHECK          Test heap consistency after every heap
                          operation; used when debugging
