[Makefile] Introduce `DESTDIR` for `make install` target

[iamazeem]

make install DESTDIR=<PATH> will make it easier to switch the installation target other than the configured prefix.
For example, for a root owned prefix such as /usr, it would be desirable to to route the installation to a non-root directory.
It would be helpful especially in cases where packages have separate configure, build, and install stages.

Reference: https://www.gnu.org/prep/standards/html_node/DESTDIR.html

[liquidaty]

So long as DESTDIR is being used in a conventional / usual / expected manner (e.g. iirc if PREFIX = /usr/local and DESTDIR = /hello/there then the installation dir will be /hello/there/usr/local) then sounds good to me