Unfortunately, this requires us to add a en-US lingua (because
we can't create a C directory without breaking out-of-tree builds).

One can now do:

    mkdir build-tree && cd build-tree
    ../user-manual/configure --with-tor-browser-bundle=../tor-browser-bundle
    make html

Easy part was sprinkling Makefile.am with appropriate $(srcdir). Symlinks
to the media needs to be done to localized version or to C version as a
fallback.

Less funny was how to deal with <xi:include /> for the version number.
We use the `-p` option of yelp-build to specify a search path. Works great.
But there's no such option to `itstool` and it errors out if it's unable
to find `type="text"` includes (to get include them in translation strings, I
presume). So we now include the Tor Browser version in an XML file instead.

Gotcha: yelp rules uses the presence of the C directory to detect if it's
an out-of-tree build. So we must not create a C directory when building.

Let's also re-add a missing variable substitution in configure.ac.

According to Mallard's specification:

    Sort titles allow you to specify alternate text to be used when sorting the
    page or section. Certain automatic link sections will sort the nodes they
    link to. When this happens, providing a sort title allows you to control
    how the node is collated. This is useful for excluding leading articles
    such as “an” and “the”.

The order of topic pages on the main index is better controlled by using
grouping capabilities of automatic links.