[v3] fix autoconf examples for testing C++11 support

[Jonathan Wakely]

The autoconf examples at
http://gcc.gnu.org/onlinedocs/libstdc++/manual/backwards.html#id541888
are incorrect, relying on pre-standard semantics for rvalue
references, and missing out several C++11 headers.  This patch
corrects those examples.

This also tweaks some markup in the file, and adds some xml:id
attributes to ensure stable anchors when the HTML is regenerated, so
the link above will become
http://gcc.gnu.org/onlinedocs/libstdc++/manual/backwards.html#backwards.third.support_cxx11
and stay the same in future.

        * doc/xml/manual/backwards_compatibility.xml: Fix autoconf tests for
        C++11 compiler features and library headers. Add stable id
        attributes. Use <filename> element for headers and surround in angle
        brackets. Use <classname> for classes.
        * doc/html/*: Regenerate.

Committed to trunk.
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