fix problem building documentation with xsltproc >= 1.1.27

[andreasbuhr]

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 content, 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.

[hkaiser]

I updated the boostbook files from the current Boost version instead of merging your request. Please verify that your problem is solved.

[hkaiser]

Hmm, this might not have been the best idea as now the doxygen generated reference refers to the header files using their full path - bad (see here: http://stellar-group.github.io/hpx/docs/html/index/s06.html).
Any idea what's going on here?

[andreasbuhr]

Yes, I figured out what the problem is:
In the old version of boostbook you had to specify what should be removed from the path. Now you have to specify what your path should start with. I'll create a new pull request with a new fix on top of the current master.