cmake: Allow building without Luajit

By default Neovim searched a Luajit instalation and linked against
the luajit library.

In practice Neovim only requires luajit to run the unit tests. All other
targets only require lua and the correct lua modules. This commit:

1. Remove the strict dependency on Luajit
2. Makes the unittest target depend on the lua 'ffi' module.
   If the module is not available the target is not enabled
   and a message is displayed.

src/nvim/CMakeLists.txt

@@ -231,7 +231,6 @@ endif()
 list(APPEND NVIM_LINK_LIBRARIES
   ${LIBUV_LIBRARIES}
   ${MSGPACK_LIBRARIES}
-  ${LUAJIT_LIBRARIES}
   ${LIBVTERM_LIBRARIES}
   ${LIBTERMKEY_LIBRARIES}
   ${UNIBILIUM_LIBRARIES}