The next official VVVVVV build removes 32-bit Linux (like all my other games),
and I need to get rid of the shell script on macOS at some point, so this
basically brings it up to what my other games are doing. Plus, this probably
fixes a bug where someone tries to run their executable away from the root...
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.
