Improve CMake installation

[Holzhaus]

I created an AUR packagei for easy installation on Arch Linux: https://aur.archlinux.org/packages/steamvr-openhmd-git/

In the process, I noticed that the installation process could use some improvement. make install does not work properly, which is why I needed to use install to copy files by hand.

Ideally, make install should be used to copy bin, resources and the driver.vrdrivermanifest file to /usr/lib/steamvr/openhmd or something like that. Then, users just create a symlink to their SteamVR installation and everything works, e.g.:

$ ln -s /usr/lib/steamvr/openhmd ~/.steam/steam/steamapps/common/SteamVR/drivers/openhmd