more flexible Makefile

[landroni]

I'm currently working on debianizing ipad-charge and eventually setting up a PPA for daily builds and I've run into an issue with the Makefile. As it is, it is not possible to install to custom locations, something that breaks down Debian packaging scripts.

I would suggest using something along the lines of:
prefix = /usr
prefix_config = /etc
bindir = $(prefix)/bin
udevdir = $(prefix_config)/udev/rules.d/

install: ipad_charge
install -m 755 -o root -g root -d $(bindir)
install -m 755 -o root -g root -d $(udevdir)
install -m 755 -o root -g root ipad_charge $(bindir)/
install -m 644 -o root -g root 95-ipad_charge.rules $(udevdir)/
install -o root -g root -m 755 ipad_charge
install -o root -g root -m 644 95-ipad_charge.rules /etc/udev/rules.d/

This would allow to:
make --prefix=debian/usr --prefix_config=debian/etc install

What do you think?