Use accessor functions to manipulate xmlOutputBuffer

Use accessor functions to manipulate xmlOutputBuffer

This is a fix to prepare the xmlj_io.c file of gnu classpath to a coming
API change in libxml2.

Basically, we were previously accessing fields inside the
xmlOutputBuffer struct of libxml2.  In a coming version of libxml2,
that won't be possible anymore.  Client code will have to use accessor
functions instead.  For the gory details, there is an interestin note
of Daniel Veillard (author of libxml2) at
https://mail.gnome.org/archives/desktop-devel-list/2012-August/msg00007.html.

This patch defines too accessor macros that, depending on the version
of libxml2 we are using will either access the fields of
xmlOutputBuffer directly, or use the new accessor function.

Tested on x86_64-unknown-linux-gnu against trunk.

libjava/classpath/

	* native/jni/xmlj/xmlj_io.c (GET_XML_OUTPUT_BUFFER_CONTENT)
	(GET_XML_OUTPUT_BUFFER_SIZE): New macros.
	(xmljOutputWriteCallback): Use them.

From-SVN: r190249