CMake 3.16

[David Chisnall]

Hi all,

At some point, I'd like to move libobjc2 to requiring at least CMake 
3.16.  3.15 gained support for driving the GCC-flavoured clang on 
Windows with the Visual Studio ABI (older versions have to use clang-cl, 
which takes Visual Studio-compatible arguments).  3.16 gained native 
support for building Objective-C[++].

Both of these changes will reduce the complexity of the libobjc2 build 
system.  Currently, I have 3.17.2 on Windows and FreeBSD, installed from 
choco on Windows and the default package system on FreeBSD, but I am 
aware that other platforms are less good at updating developer tools.

What is the most recent cmake that is easy to install on your GNUstep 
development systems?  I'd like to get an idea of when moving to 
depending on 3.16 (released last November) will be viable for most people.

David