[config] configure.ac checks required features, updated porting

Added a real config.rpath
Fixed build date calc for BSD
Require some needed headers at configure time
Correctly check libav functions using pkg-config flags
Made checks for some Linux features available on other ports
Fixed porting issue with antlr dep files
Added default HOST_NAME_MAX if not defined
Added missing libav header

src/mdns_avahi.c

@@ -630,6 +630,11 @@ entry_group_callback(AvahiEntryGroup *g, AvahiEntryGroupState state, AVAHI_GCC_U
     }
 }
 
+#ifndef HOST_NAME_MAX
+/* some systems want programs to query this, just define a reasonable limit */
+#define HOST_NAME_MAX 64
+#endif
+
 static int
 create_group_entry(struct mdns_group_entry *ge, int commit)
 {