Allow to build ROOT without any Internet connection

It should be allowed to build ROOT on an offline computer. Currently this is not possible since the cmake building system try to download some software from the Internet, such as LZ4, tbb and maybe others.

Problems of such behaviour are described here. In summary, the build system must not supposed that the computer is connected to Internet. If the package is not found on the system, the cmake step has to inform the user that such package is not present on the system.