Explicitly set CCCL_TOPLEVEL_PROJECT to OFF when needed (#7016)

Author: KyleFromNVIDIA

CMakeLists.txt

@@ -9,8 +9,15 @@ endif()
 
 # Determine whether CCCL is the top-level project or included into
 # another project via add_subdirectory()
-if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
-  set(CCCL_TOPLEVEL_PROJECT ON)
+
+if (NOT DEFINED CCCL_TOPLEVEL_PROJECT)
+  # DO NOT REMOVE THE FOLLOWING LINE
+  set(CCCL_TOPLEVEL_PROJECT OFF)
+  # REQUIRED FOR CCCL TO WORK VIA CPM WITH INSTALL RULES
+
+  if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
+    set(CCCL_TOPLEVEL_PROJECT ON)
+  endif()
 endif()
 
 # Enable CXX so CMake can configure install paths