Fixed some CMake Bugs related to non-debian distros

This [greatly simplifies](https://github.com/bwrsandman/pkgbuild/commit/69b96667fe482032fa06860a61e9e1d4dbb2c466) the custom packaging needed in Arch Linux by allowing the use of `$ make install` which was broken prior to this.

I got rid of a bug that installed the application data (.desktop files) in `/share` instead of `/usr/share`.
I put openmw data files in `/usr/share/games/openmw`.

I don't think the change affects anyone who is not using `$ make install` on a non-debian machine.