install.sh is outdated/buggy

[lxkurko]

I'm not sure if anyone uses the install.sh script, but it doesn't seem to work as it should. For example, if you set SYSTEM_DATA_DIR=/usr/local/share/games in CMake configuration, the script still installs the game executable into /usr/bin/ and the game data into /usr/share/OpenLieroX/, and this causes the game to exit on startup because it can't find the game files.

Tested on Debian Stretch with OpenLierox 0.58 from Git.