CMake: Set luajit EXCLUDE_FROM_ALL;

This prints a warning but it's nice to not build the luajit executable.

Ideally the target would not be created at all but the CMakeLists we
are using does not expose this as an option.

This is congruent with the vanilla Lua build options.

CMakeLists.txt

@@ -119,6 +119,7 @@ if(LOVR_USE_LUAJIT AND NOT EMSCRIPTEN)
     set(LOVR_LUA ${LUAJIT_LIBRARIES})
   else()
     add_subdirectory(deps/luajit luajit)
+    set_target_properties(luajit PROPERTIES EXCLUDE_FROM_ALL 1)
     include_directories(deps/luajit/src ${CMAKE_BINARY_DIR}/luajit)
     set(LOVR_LUA libluajit)
   endif()