This was annoying to figure out because the Inkscape manpage still shows
--export-png as a valid option, even though it was removed in the last
release. It has to be --export-file now:

https://wiki.inkscape.org/wiki/index.php/Using_the_Command_Line#New_options

Undocumented features…

Since I still use this Makefile for my own blog featured images, I
figured I should go ahead and upstream my patch. Thanks @pfrields and
@ryanlerch for this handy little script:

https://github.com/jwflory/blog-images/commit/937c3d3627e82536cdc31dc73af6aa50d2108d51