Having GST_DEBUG_CATEGORY and GST_DEBUG_CATEGORY_EXTERN together
might lead to 'undefined symbol' problems. This commit moves
the _EXTERN to a separate new file.
This uses same approach like in playbin, namely checking for user defined
element, auto{audio,video}{sink,src} and finally DEFAULT_{AUDIO,VIDEO}{SRC,SINK}
defines from config.h.
Windows doesn't have strcasecmp so we should use
g_ascii_strcasecmp here instead.
Also Windows doesn't have sys/time.h so we first
need to check if it actually exists and if not
we should not include it.
Fixes bug #572315.
