Makefile: Add PREFIX support

Default to "/usr/local" which should be used if you want to build
devtools from sources and install without using a package manager.

Use `make PREFIX=/usr install` to install to "/usr" instead.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>