Define PATH_MAX ourselves if it isn't defined by the OS (closes #90)

This is kinda sloppy coding (feh --filelist and --bg-* will fail when running
on a system with path lengths >= 4096 and PATH_MAX not defined), but that's
sufficiently improbable to be okay. (plus, I ain't getting paid for this, so
if you want to improve it feel free to do so)