Since the ebuild hasn't made it in the main portage tree or the games overlay yet, i've fixed a few issues and added it to the arx-libertatis overlay.

One thing i've noticed is that flare's CMakeLists.txt explicitly prepents ${CMAKE_INSTALL_PREFIX} to the path in install command. This is not necessary for relative paths but prevents using absolute paths for BINDIR (or DATADIR) like GAMES_BINDIR provided by the games eclass in gentoo or %{_bindir} in rpm spec files.

You would then also need to adjust the FLARE_EXECUTABLE_PATH:

if(NOT IS_ABSOLUTE ${BINDIR})
  set(FLARE_EXECUTABLE_PATH ${CMAKE_INSTALL_PREFIX}/${BINDIR}/flare)
else()
  set(FLARE_EXECUTABLE_PATH ${BINDIR}/flare)
endif()

or just assume that flare is in the $PATH and use that in flare.desktop