[phobos] How to build the Phobos docs?

[Lars Tandle Kyllingstad]

When I run "make -f posix.mak html" I now get the message

         make: *** No rule to make target
        `../web/phobos-prerelease/phobos.html', needed by `html'. Stop.

If I create the file manually, I get more of the same:

        make: *** No rule to make target
        `../web/phobos-prerelease/std_algorithm.html', needed by `html'.
        Stop.

Is the makefile broken, or is there something I need to do before
running make?

-Lars