Add support for installing compilers to XDG_DATA_HOME

[dweiller]

This PR add a build option -Duse-xdg that enables use of XDG_DATA_HOME for the installation directory on non-windows operating systems.

This PR conflicts with #141 - whichever gets merged first will cause minor fixups to be needed in the other.

Before merging:

• [ ] add tests to CI for binaries built with -Duse-xdg