Installation paths are broken unless default is used
I've noticed two issues with the current way the installation paths are handled. These are important for Linux distro packagers as each distribution has rules around where specific files should be placed. Let me know what you think of these and if you'd like me to put up a PR for these suggestions.
Non default PREFIX breaks shared script usage
Currently the Makefile allows for using a custom PREFIX for installation, and this is good. However changing the PREFIX say to /usr, so the main script ends up in /usr/bin, means that the helper scripts will land in /usr/share/reddio, which is also correct behaviour.
However the default library path is hardcoded in reddio:184 to /usr/local/share/reddio. This means that out of the box reddio installation will not work when PREFIX was changed, unless the user uses the command with the REDDIO_LIB environment variable or create a configuration in $HOME/.config first where $lib_dir is set. This is not a desirable outcome as the code should work out of the box post installation.
I'd like to propose replacing the hardcoded value with a system wide default config in /etc/xdg/. The actual path for the directory is available in [XDG_CONFIG_DIRS](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html). During installation the Makefile could do `echo "lib_dir=\"{PREFIX}/${LIB_FOLDER}"" > /etc/xdg/reddio. Then the script would check in order of preference the REDDIO_LIB` environment variable, the user's home directory config, and finally the system wide config.
This would make an arbitrary installation work out of the box (e.g. what if a user prefers "internet installations" into /opt or similar) and would allow distro packagers to meet their specifications without hacking multiple files.
Rigid documentation path
Documentation path might also need to be customized (e.g. in Gentoo we require the documentation folder to have the version too, e.g. /usr/share/doc/reddio-0.46). For this a simple change would be needed by changing DOC_FOLDER assignment to ?= in the Makefile.