kafka: fix compilation for older versions of librdkafka

Since commit f505691270f2317291c372fd5f004a4ffbce9f9a, kafka module was
broken. Enable definition of `kafka_log()` when using kafka logger
callback as well.
Merge pull request #794 from vincentbernat/fix/kafka-warning

kafka: fix compilation for older versions of librdkafka
Define _DEFAULT_SOURCE in addition to _BSD_SOURCE

This enables forward compatibility with the ongoing
deprecation of _BSD_SOURCE.
ignore new dirs