html: Set CONTENTS_OUTPUT_LOCATION=inline if makeinfo supports it

This flag allows us to restore old (pre-6.8) behavior of the
@{summary,}content commands, so that texi2any continues to emit
summarycontents first.

maintainer-scripts/ChangeLog:

	* update_web_docs_git: Set CONTENTS_OUTPUT_LOCATION=inline in
	order to put @shortcontents above contents.

gcc/ChangeLog:

	* configure.ac: Add check for the Texinfo 6.8
	CONTENTS_OUTPUT_LOCATION customization variable and set it if
	supported.
	* configure: Regenerate.
	* Makefile.in (MAKEINFO_TOC_INLINE_FLAG): New variable.  Set by
	configure.ac to -c CONTENTS_OUTPUT_LOCATION=inline if
	CONTENTS_OUTPUT_LOCATION support is detected, empty otherwise.
	($(build_htmldir)/%/index.html): Pass MAKEINFO_TOC_INLINE_FLAG.