Bring back standalone functionality, but only when the library is the root project

CMakeLists.txt

@@ -1,5 +1,6 @@
 # Generated by `boostdep --cmake sort`
 # Copyright 2020, 2021 Peter Dimov
+# Copyright 2025 Nigel Stewart
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
@@ -21,8 +22,25 @@ target_link_libraries(boost_sort
     Boost::type_traits
 )
 
-if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
+if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
 
-  add_subdirectory(test)
+  message(STATUS "Using CMake version ${CMAKE_VERSION}")
+
+  set(Boost_DEBUG ON)
+  find_package(Boost 1.85 CONFIG REQUIRED COMPONENTS core range included_test_exec_monitor)
+
+  include(CTest)
+
+  add_custom_target(check COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure -C $<CONFIG>)
+  add_dependencies(check tests)
+
+endif()
+
+# Testing
+if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
+
+  # Follow the Boost convention: don't build test targets by default,
+  # and only when explicitly requested by building target tests
+  add_subdirectory(test EXCLUDE_FROM_ALL)
 
 endif()