Fixed default install location

Originally broken by addition of Debian package support.

The default installation direction is actually /usr at present despite
what 'INSTALL' says.

The 'debian/rules' makefile specifies the prefix as /usr so doesn't
seem to depend on this. I've tested the resulting debian package and
everything is still installed correctly under /usr.

Signed-off-by: Daniel White <daniel@whitehouse.id.au>
Signed-off-by: Karl Hasselström <kha@treskal.com>