Disable installation without the superproject; add comments

CMakeLists.txt

@@ -1,9 +1,12 @@
 # Copyright 2018 Mike Dev
+# Copyright 2019 Peter Dimov
 # 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)
-project(BoostStaticAssert LANGUAGES CXX)
+# We support CMake 3.5, but prefer 3.16 policies and behavior
+cmake_minimum_required(VERSION 3.5...3.16)
+
+project(boost_static_assert VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
 
 add_library(boost_static_assert INTERFACE)
 add_library(Boost::static_assert ALIAS boost_static_assert)
@@ -14,3 +17,13 @@ target_link_libraries(boost_static_assert
     INTERFACE
         Boost::config
 )
+
+# boost_install requires PROJECT_VERSION
+# Without the superproject, we don't have any, so skip installation
+
+if(BOOST_SUPERPROJECT_VERSION)
+
+  include(BoostInstall)
+  boost_install(TARGETS boost_static_assert HEADER_DIRECTORY include/)
+
+endif()