Fix undefined PROJECT_IS_TOP_LEVEL for cmake <3.21

Author: StevenvdSchoot

test/CMakeLists.txt

@@ -2,6 +2,18 @@ cmake_minimum_required(VERSION 3.15...3.23)
 
 project(CmakeConfigPackageTests LANGUAGES CXX)
 
+# This variable is set by project() in CMake 3.21+
+string(
+  COMPARE EQUAL
+          "${CMAKE_SOURCE_DIR}"
+          "${PROJECT_SOURCE_DIR}"
+          PROJECT_IS_TOP_LEVEL)
+if(PROJECT_IS_TOP_LEVEL)
+  # Consider the CTest module, which creates targets and options!
+  # Only needed if you want to enable submissions to a CDash server.
+  include(CTest)
+endif()
+
 # ---- Test as standalone project the exported config package ----
 
 if(PROJECT_IS_TOP_LEVEL OR TEST_INSTALLED_VERSION)