glibc needs _GNU_SOURCE defined to make IOV_MAX visible. Use AC_USE_SYSTEM_EXTENSIONS and make sure to include compat.h before everything else. Found by cage_, thanks!

Use asr on OpenBSD to do DNS query lookups asynchronously. At the moment it seems a bit hard to provide the whole asr + asr_event stuff in the compat layer, so hide those bits behind an #ifdef; not the prettiest solution, but a working one. Future works may either: - provide a stub implementation for the asr stuff - bundle the libasr from OpenSMTD-portable