Update required CMake version to 3.8 (#1088)

Most libraries are C++11 and require CMake 3.8 for the CMake feature or
depend on libraries that do.
Drop support for earlier CMake versions in the super project to reduce
the number of versions to verify.

CMakeLists.txt

@@ -2,7 +2,7 @@
 # Distributed under the Boost Software License, Version 1.0.
 # See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.5...3.16)
+cmake_minimum_required(VERSION 3.8...3.16)
 
 # The default build type must be set before project()
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)