Makefile: allow custom base path to install
for example, `make DESTDIR=/usr install`.
ROOTDIR=$(dir $(abspath $(lastword $(MAKEFILE_LIST))))
# Base path used to install.
-DESTDIR=/usr/local
+DESTDIR ?= /usr/local
# Used to populate variables in version package.
VERSION=$(shell git describe --match 'v[0-9]*' --dirty='.m' --always)
