Add DESTDIR to Makefile

This PR is targeted at `feature/add-pkgconfig` since that branch is required but if this works, it would be nice to submit upstream. 

[`DESTDIR`](http://www.gnu.org/prep/standards/html_node/DESTDIR.html) is useful for package managers like [pacman](https://wiki.archlinux.org/index.php/pacman) that use a temporary root when building and necessary for the `zlog.pc` file to be generated correctly with [makepkg](https://wiki.archlinux.org/index.php/makepkg).

Without this change, the AUR [zlog-git](https://aur.archlinux.org/packages/zlog-git/) package pointing at `feature/add-pkgconfig` generates a bogus `zlog.pc` unless some `sed` is used to fix the problem. With this change, and modifying the package to use `DESTDIR` all is well.

NOTE: I have not tested this on other platforms/package managers