[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/transcode.c

@@ -32,6 +32,7 @@
 #include <libavutil/opt.h>
 #include <libavutil/time.h>
 #include <libavutil/pixdesc.h>
+#include <libavutil/channel_layout.h>
 
 #ifdef HAVE_LIBAVFILTER
 # include <libavfilter/avcodec.h>