Always use $prefix in directory names.
    
    Hardcoding absolute paths breaks the distcheck target, since it uses
    --prefix instead of $DESTDIR to change the installation destination. For
    files in /etc, we can use $sysconfdir and rpmbuild will set it to
    /etc instead of $prefix/etc. For the gi overrides directory, it looks like
    other projects just assume that it's under $pyexecdir, which it is.