Before OS X 10.6, defining BIND_8_COMPAT included arpa/nameser8_compat.h
which was removed in 10.6 and from then on included arpa/nameser_compat.h
until BIND_8_COMPAT was removed somewhere between macOS 10.13 & 10.15.
On systems before 10.6 this causes the build to break due to conflicts.