Support DESTDIR on Makefile

Gentoo will install this script by default using the following command: emake DESTDIR="${D}" install however the Makefile currently make no use of this variable.

It would be nice if the Makefile could use this variable instead of enforcing the binary location of /usr/bin/local.

Ref:

• https://www.gnu.org/prep/standards/html_node/DESTDIR.html
• https://devmanual.gentoo.org/ebuild-writing/functions/src_install/index.html