Move dev build toggle to the cccl project level.

This allows the install rules to be generated for all projects even
when the dev builds are disabled.

CMakeLists.txt

@@ -6,8 +6,7 @@ project(Thrust LANGUAGES NONE)
 
 # Determine whether Thrust is the top-level project or included into
 # another project via add_subdirectory()
-if (CCCL_TOPLEVEL_PROJECT OR
-    "${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
+if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_LIST_DIR}")
   set(THRUST_TOPLEVEL_PROJECT ON)
 endif()