Makefile: make inkscape export rule compatible with both Inkscape 0.xx and 1.xx

The Inkscape command format for export changed in Inkscape 1 and was updated in 2ea63aae

Some users still use Inkscape 0.

I made the export rule compatible with the two possible versions, calling the right command depending on the version of Inkscape.

@tarik : can you test with Inkscape 1, I only have Inkscape 0.