dmx: Construct paths in doxygen.conf to fix VPATH builds

[Sedat Dilek]

Hi,

> I also have rename detection on here to make the reviewing easier, so
> you'll need git am or apply to do the right thing.

applying the patch from [1] here with git-1.7.4.1 via "git apply" fails.
The renaming part "hw/dmx/doc/{doxygen.conf => doxygen.conf.in" fails
and doxygen.conf file was deleted.
Instead I had success via "git am".

Feel free to add a...

Tested-by: Sedat Dilek <sedat.dilek at gmail.com>

- Sedat -

[1] http://patchwork.freedesktop.org/patch/4193/