The patch datadir.patch makes warzone2100 use /usr/share/games/warzone2100/
as the directory for data files (base.wz etc).  I expect that there is some
way of passing a parameter to cmake to get this result without a code change.

I've tried setting -DWZ_DATADIR=/usr/share/games/warzone2100/ but that
resulted in "cmake install" failing due to lack of write access to /usr.

This is probably an easy problem to solve for someone who is good at cmake.