Allow checking if CMake has SSL support

It would be useful to allow checking if CMake was built with SSL support. I received several complaints that https download failed, caused by manually compiled CMake, without SSL support.

This could be a simple variable, i.e. something like

if (NOT CMAKE_HAS_SSL_SUPPORT)
  message(FATAL_ERROR "...")
endif()

or, eventually in the cmake_minimum_required command, for example

cmake_minimum_required(VERSION 3.18 FEATURES SSL)