CTest: Module must be included at the top level

In order to be able to run `ctest` from the top level build directory,
it is necessary that `enable_testing()` is invoked in the top level
CMakeLists.txt file. The `CTest` module invokes `enable_testing()`
based on the value of `BUILD_TESTING`.

Author: purpleKarrot

CMakeLists.txt

@@ -10,6 +10,12 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
 endif()
 
+option(BUILD_TESTING "Build the testing tree." ON)
+if(BUILD_TESTING)
+  enable_testing()
+endif()
+
+include(CTestUseLaunchers)
 include("cmake/compat_find.cmake")
 
 find_package(CapnProto REQUIRED)

test/CMakeLists.txt

@@ -2,8 +2,6 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-include(CTest)
-
 # Custom test targets for convenience, based on
 # https://gitlab.kitware.com/cmake/community/-/wikis/doc/tutorials/EmulateMakeCheck.
 #