log4tango: Use BUILD_TESTING option
We don't want to determine that by the platform. Accompanying fix to a81c1584 (CMakeLists.txt: Rework test suite handling, 2020-01-23).
We don't want to determine that by the platform. Accompanying fix to a81c1584 (CMakeLists.txt: Rework test suite handling, 2020-01-23).