decode: add some flags for windows

We are doing a static compile of libyaml, so set the
YAML_STATIC_DECLARE compile macro. Also, strdup
is only available in the common run time (CRT) as
_strdup, so use another macro for that.

We could do it in other ways if there is something
more appropriate for the yaml library. Originally I
was doing it in an #ifdef macro, but this makes it
nice and minimal and doesn't touch the yaml files
directly.

R=
CC=
https://codereview.appspot.com/7719045