libqtxdg fails to build on darwin.
LXQt is a DE for linux so assume darwin is not supported.
The platform for some packages that may be useful outside of
LXQt (like pcmanfm-qt, lximage-qt and others) and do not depend on
libqtxdg are not being changed.
LXQt uses the values in this macro to set installation
directories and in the case of lxqt-session to substitute in paths.
However, by doing this all the paths are from within lxqt-build-tools
prefix. By utilizing a setup hook we can set the directories with the
correct prefix as cmake flags.
