makefile: auto-detect presence of various Lua, bsd

We favor LuaJIT over Lua. We disable Lua if neither can be found. We
error out if a particular Lua is specified via LUA_IMPLEMENTATION=JIT or
LUA_IMPLEMENTATION=VANILLA, but cannot be found. We print a status
message depending on what happens.

Also, we do not link against libdl on the BSDs, since they include it as
part of libc.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>