My understanding is that ‘BUILD_TESTING’ is not standard, as the last
sentence above suggests.  Thus I’m reluctant to passing this flag
unconditionally, as I guess it would fail for ‘CMakeLists.txt’ that do
not implement it, right?

I just tested it, and all you get is a warning:

$ cat CMakeLists.txt 
cmake_minimum_required(VERSION 3.0)
project(doo)
$ cmake -DBUILD_TESTING=OFF .
…
-- Generating done (0.0s)
CMake Warning:
 Manually-specified variables were not used by the project:

   BUILD_TESTING


-- Build files have been written to: /tmp/xxx


--

Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |