Patch for dia support

Hello,

   We use mscgen, dot and dia for generating/drawing graphs for
documentation. Doxygen can use mscgen and dot files directly, so for dia
we've been manually exporting bitmaps and eps files. It was fun the
first 43957395 times, but I started getting tired of it, so I added a
\diafile keyword to doxygen.

   The patch is available at:
https://bugzilla.gnome.org/show_bug.cgi?id=707713

   Feedback wanted.

   For some reason the pre-built Mac OS X version of dia won't accept
command line arguments. I built dia from pkgsrc so I could use \diafile
on my Mac as well.

   /Jan

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