Bumping CMake required version and adding warning message.

CMakeLists.txt

@@ -1,6 +1,14 @@
-cmake_minimum_required(VERSION 3.17)
+cmake_minimum_required(VERSION 3.21)
 
 project(History-Output VERSION 0.0.1 LANGUAGES Fortran)
+
+if(History-Output_IS_TOP_LEVEL)
+  message(WARNING "History-Output is not integrated into the CMake build of any top level "
+                  "project yet and this CMake is for testing purposes only.  "
+                  "Making a change to this project's CMake will not impact the build of "
+                  "a parent project at this time.")
+endif()
+
 option(HISTORY_ENABLE_TESTS "Run pFUnit unit tests" OFF)
 option(HISTORY_ENABLE_CODE_COVERAGE "Run code coverage tool" OFF)