Fix OOS build with 'lua_source'

The origin was that 'vmdef.lua' was generated in
'${CMAKE_BINARY_DIR}/third_party/luajit/jit/', but was searched in
'${CMAKE_SOURCE_DIR}/...'.
ALSO: when we start to generate *.c files from luajit's libraries we made
statement, that we don't built with OS pre-built luajits.