-DDATADIR has no effect

[mcz]

What I would expect to happen:

Data files get installed to ${CMAKE_INSTALL_FULL_DATADIR}/openxcom
The game searches the following locations for data files:

• $XDG_DATA_HOME/openxcom (if $XDG_DATA_HOME is set)
• $HOME/.local/share/openxcom (if $XDG_DATA_HOME isn't set)
• ${DATADIR}
• $XDG_DATA_DIRS/openxcom (if $XDG_DATA_DIRS is set)
• usr{,/local}/share/openxcom (if $XDG_DATA_DIRS isn't set)
• ${CMAKE_INSTALL_FULL_DATADIR}/openxcom
• ./

What actually happens:

Data files get installed to ${CMAKE_INSTALL_FULL_DATADIR}/openxcom
The game searches the following locations for data files:

• $XDG_DATA_HOME/openxcom (if $XDG_DATA_HOME is set)
• $HOME/.local/share/openxcom (if $XDG_DATA_HOME isn't set)
• $XDG_DATA_DIRS/openxcom (if $XDG_DATA_DIRS is set)
• usr{,/local}/share/openxcom (independently of $XDG_DATA_DIRS)
• ${CMAKE_INSTALL_FULL_DATADIR}/openxcom
• ./

The two commits responsible are #474 which added ${DATADIR} and also put usr{,/local}/share/openxcom on the directory list independently of the state of $XDG_DATA_DIRS, and #1057, which then replaced ${DATADIR} ${CMAKE_INSTALL_FULL_DATADIR}/openxcom causing ${DATADIR} to go unused.

I should also mention that while the desription of ${DATADIR} is wrong, so was my understanding of it. It is described as "Where to place datafiles", while I thought it meant "Where to look for datafiles".

($VAR means run-time environment variable, ${VAR} means build-time CMake variable)