flac: blacklist clang older than build 503

Fixes #34725.
net-snmp 5.7.3
net-snmp: enable compat headers and libs
net-snmp: keg-only
lldpd 0.7.12
nut: bump revision for net-snmp changes
powerman: bump revision for net-snmp changes
lldpd: add 0.7.12 bottle.
net-snmp: update 5.7.3 bottle.
nut: update 2.7.2_1 bottle.
powerman: update 2.3.20_1 bottle.