Follow-up to #19 which did the change for macOS.
It appears to work as expected on Linux too.
Tested on a distro where XDG_DATA_HOME is undefined by default,
and `PHYSFS_getPrefDir` also resolves to `~/.local/share/VVVVVV/`.
The first organization parameter is unused on Linux and macOS.
Instead use PHYSFS_getPrefDir which does the same than the manual
concatenation done before. The organization name argument is required
but is not used on macOS.
