Support platform specific INPUTDIR

I wrote blog posts on both my Mac (main computer) and Windows desktop (at the office). I need a different INPUTDIR for each, due to the different folder structures on the two computers. This change allows you to create an INPUTDIR_osx, INPUTDIR_windows, and (presumably) INPUTDIR_linux setting in the Makefile, to have a different folder for each platform. If those settings don't exist, it will fall back to the default INPUTDIR.