allow dxf deps writing

From the mailing list 1/16/2013:
post by W. Craig Trader

quote
"
The source file is here:  

    https://github.com/wcraigtrader/game-parts/blob/master/population_storage_tray.scad
. . . 
    $ make
    openscad -m make -o population_storage_tray.dxf -d population_storage_tray.dxf.deps population_storage_tray.scad
"

it resulted in a crash at the write_deps() call. 

This patch fixes by properly detecting dxf_output file and feeding as appropriate. 

I tested it and the dxf appears to be properly generated but the patch could definitely use some outside verification