Fix undefined PROJECT_IS_TOP_LEVEL for cmake <3.21

Author: StevenvdSchoot

test/CMakeLists.txt

@@ -4,6 +4,13 @@ project(CmakeConfigPackageTests LANGUAGES CXX)
 
 # ---- Test as standalone project the exported config package ----
 
+# 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 OR TEST_INSTALLED_VERSION)
   enable_testing()