#383 respect DESTDIR env variable during installs

[Martin Mokrejs]

The installation process should respect user's wish to install files for example into /scratch while the files can then be moved by root user over to a /. This is the typical installation procedure on Gentoo Linux (not to install apps dsirectly as root but as a non-privileged user and move the files over to final destination after some extra checks, file permission checks, etc. There are 3 patches I have already sent to the list a while ago:

https://sourceforge.net/mailarchive/message.php?msg_id=27484193

I am not re-attaching them here as they can be extracted directly from mailing list archives.