Explicly define NI_MAXHOST and NI_MAXSERV

They should come with <netdb.h> if the appropriate feature test macros
are defined. But defining the right ones (e.g. _POSIX_C_SOURCE with
value 200112L or even 200809L), ideally in configure.ac, requires more
investigation, given that different macros are spread around the
codebase.

This change is aimed at quickly fixing the build on macOS.

src/socklib/ipv6sock.cc

@@ -29,6 +29,13 @@
 #  define _POSIX_SOURCE 1
 #endif
 
+#ifndef NI_MAXHOST
+#  define NI_MAXHOST 1025
+#endif
+#ifndef NI_MAXSERV
+#  define NI_MAXSERV 32
+#endif
+
 #include <sys/types.h>
 #include <netdb.h>
 #include <sys/socket.h>