Revert change to function.

Will implement using language attribute instead.

[SVN r86508]
Add support for language='c++'.

The implementation is a bit odd, but I was trying to match the existing
templates.

[SVN r86509]
Use language attribute in doxygen2boostbook.

It would be better to preserve doxygen's syntax highlighting, but I'll leave
that for later.

[SVN r86510]