Neovim fails to build with the latest luaJIT commits because it cannot find a compatible luaJIT runtime. @algent mentioned that Arch uses luaJIT 2.1.0, but they're using a beta tag that's as old as the last stable release, so I suspect something has changed somewhere along the way in the luaJIT project.

I think I found what happened with luaJIT to cause neovim build to break. The binary's name changed. Or rather, a link was removed. Before, there was a file (possibly a link) to the versioned binary that was just named luajit. The new version doesn't have that, and Arch adds a symlink in their package to get the same behavior. Neovim uses find_program to find the lua runtime, and it just looks for luajit, so not having the link causes the check to fail.

Note: solarus-based games crash on startup with an error "Non-empty Lua stack", but it's fixed with the latest commit of LuaJIT (that's pulled in the linked diff).

After merging the related differentials both solarus and solarus-quest-editor will work again.
solarus games broke after the update of LuaJIT, but the new synced commit fixes them. solarus-quest-editor was apparently broken for a long time (its crash is completely unrelated to Lua), but with a backported patch it's functional again.