CMake minimum version to support

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-06-02 16:32:05

I suppose the time has come to have the discussion on
the minimum CMake version we will support for building
Boost. Currently that's set to 3.5, which kind of works, but
is fairly inconvenient.

The interesting options are

- CMake 3.8: required for use of the compile features
  cxx_std_11, cxx_std_14, cxx_std_17, etc. Also allows
  source_group(TREE, which is used by Json and
  PropertyTree.

- CMake 3.9: required by Nowide, I'm not sure why.
  Also, required for FindMPI to define an imported
  target. Building Boost.MPI is optional though.

- CMake 3.13: required for Boost installation support.

- CMake 3.14: required for FindPython to be modern
  enough to make building Boost.Python reasonably
  easy. (Building Boost.Python is optional, though.)

There are also less important nice-to-have things such as

- CMake 3.10: adds include_guard()
- CMake 3.12: adds CONFIGURE_DEPENDS to file(GLOB
- CMake 3.15: adds VERBOSE/DEBUG to message(

Opinions?