Makefile: don't hardcode the path to 'install'

In Arch Linux we currently use /bin/install rather than
/usr/bin/install. This change work with both locations and
avoids us having to change the Makefile.

Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>