Staged installs support.

Some package managers (f.e., gentoo portage) require staged install support:
i.e. installation of files into some temporary location (prefix).
This changeset adds support of that feature via DESTDIR variable.
Also see https://www.gnu.org/prep/standards/html_node/DESTDIR.html .

Related haxelib commit: https://github.com/HaxeFoundation/haxelib/pull/128