Cosmetic update to Makefile to avoid errors on purge

This should avoid errors in 'purge' by checking on which kind of system we're running, and then running the appropriate commands.
Merge branch 'patch-1' of https://github.com/roeme/amtc

merged, thanks... (like the improvement, but the patch has some issues...)
tweak roeme's patch a bit (indentation, use/doc PKGTYPE)