Easier packaging

When packaging for Linux distributions there are different approaches. For example running the `make install` command in a fakeroot environment that writes the files in a package instead of the filesystem, for this, the current install target works.
For Arch Linux (and probably others) we use a directory where all files get copied to and package that.

This addition allows setting the directory to install the directory structure to and is well known for [autotools][1] and [cmake-based][2] projects.

Another small addition is an explicit dependency, it only install files when they have been built before.

[1]: https://www.gnu.org/software/automake/manual/html_node/DESTDIR.html
[2]: http://www.cmake.org/Wiki/CMake_FAQ#Does_CMake.27s_.22make_install.22_support_DESTDIR.3F