log4tango: Use BUILD_TESTING option

We don't want to determine that by the platform. Accompanying fix to
a81c158 (CMakeLists.txt: Rework test suite handling, 2020-01-23).

Author: t-b

log4tango/CMakeLists.txt

@@ -12,6 +12,7 @@ include(${CMAKE_CURRENT_SOURCE_DIR}/config/config.cmake)
 
 add_subdirectory(include)
 add_subdirectory(src)
-if (NOT WIN32)
+
+if(BUILD_TESTING)
     add_subdirectory(tests)
-endif(NOT WIN32)
+endif()