Native CMake integration

[gabrielecastellano]

This creates a root CMakeLists that can be used to directly configure and build the project natively.

For instance:

$ mkdir cmake-cache && cd cmake-cache
$ cmake -DBUILD_SIM=ON -G "Unix Makefiles" ..
$ cmake --build . -j 103

This can be helpful for developing (code inspection features) and for future maintenance in general.

Works with PR aliyun/ns-3-alibabacloud#9 for the ns3 subproject.

(tested with simulator only)