fix problem building documentation with xsltproc >= 1.1.27

It seems variables must not be used in the "use" attribute for an xsl:key. See also 
https://bugzilla.gnome.org/show_bug.cgi?id=705865
xsltproc 1.1.27 and later seem to enforce this. This prevents Ubuntu 13.04 users to build the HPX documentation.

A quickfix is to replace the variables with their names, which is done with this patch.

The boost people did the exact same thing (replacing the variables with their content), see:
http://svn.boost.org/svn/boost/trunk/tools/boostbook/xsl/annotation.xsl

Getting the latest version from boost would be a cleaner solution. But as I cannot oversee possible side effects I propose this simple fix.