CMake: Show used CMake version

[Flamefire]

In super-builds show the CMake version which is especially useful on CI to verify the used CMake and/or compare different jobs on failure.

Limited to the super build to not be too verbose when Boost is used via add_subdirectory or similar.

[nigels-com]

Good idea! Would be helpful.

[pdimov]

1. pull requests should be against develop
2. logic such as this goes into BoostRoot.cmake, to keep CMakeLists.txt minimal

[Flamefire]

pull requests should be against develop

Can/Shall we change the default branch to develop? When opening a pull request here it defaulted to the master branch instead and I forgot to verify

[pdimov]

The default branch here is kept on master so that people doing get clone --recurse-submodules get that instead of develop which is going to be much less stable.

[Flamefire]

logic such as this goes into BoostRoot.cmake, to keep CMakeLists.txt minimal

--> boostorg/cmake#88