CMake: use enable_testing() instead of include(CTest)

According to Craig Scott in https://discourse.cmake.org/t/is-there-any-reason-to-prefer-include-ctest-or-enable-testing-over-the-other/1905/2 ,
using include(CTest) adds unnecessary clutter that is only needed for
dashboard submission. enable_testing() is enough otherwise

"Port" of OSGeo/shapelib#162

CMakeLists.txt

@@ -382,9 +382,12 @@ add_subdirectory(capi)
 #-----------------------------------------------------------------------------
 # Tests
 #-----------------------------------------------------------------------------
+
+option(BUILD_TESTING "Build the testing tree." ON)
+
 if(PROJECT_IS_TOP_LEVEL)
-  include(CTest)
   if(BUILD_TESTING)
+    enable_testing()
     add_subdirectory(tests)
   endif()
 endif()