A lot of users try to build the development version --with-system-nvtt, which is not the default, but is recommended since we match upstream. Unfortunately, a lot of package managers distribute nvtt only for 0ad releases, so they are behind, as was alpha 23. Users of the old NVTT will experience a crash on startup, which is otherwise fixed by cleaning the cache (try this first).
This patch prevents building with an old NVTT and suggests using the bundled version.