The dnsinfo.h header was extracted from Apple's OpenSource repository: https://opensource.apple.com/source/configd/configd-453.19/dnsinfo/dnsinfo.h

We then had to make a few edits to this file:

1. Add AvailabilityMacros.h header file
2. conditionalize reach_flags in dns_resolver_t on MacOS 10.8 or higher, in order to maintain compatibility with the last MacOS PPC release, 10.6.
3. conditionalize _dns_configuration_ack() on MacOS 10.8 or higher.
4. Update parameter list to (void) for both dns_configuration_notify_key() and dns_configuration_copy() to sidestep compiler warnings in this old header.

We had tried initially to use the latest 1109.140.1 which only worked on MacOS 11+, then downgraded to 963.50.8 for MacOS 10.8+ support, then finally to 453.19 with additional patches.

This is needed to call into dns_configuration_copy() and dns_configuration_free().