add_subdirectory(foo system) doesn't work for me

Usage
1

I’m trying to add system subdirectories.
my project is as follows:
CMakeLists.txt

cmake_minimum_required(VERSION 3.5.1)
PROJECT(test VERSION 0.0.0.1)
ADD_SUBDIRECTORY(b1 SYSTEM)
ADD_SUBDIRECTORY(b2 SYSTEM)
ADD_EXECUTABLE(m1 m1.cc)

b1\CMakeLists.txt

ADD_EXECUTABLE(b1 b1.cc)

b2\CMakeLists.txt

ADD_EXECUTABLE(b2 b2.cc)

And when I run cmake on that I get:

cmake ..
-- Building for: Visual Studio 17 2022
-- Selecting Windows SDK version 10.0.19041.0 to target Windows 10.0.22000.
-- The C compiler identification is MSVC 19.34.31935.0
-- The CXX compiler identification is MSVC 19.34.31935.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files/Microsoft Visual Studio/2022/Professional/VC/Tools/MSVC/14.34.31933/bin/Hostx64/x64/cl.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files/Microsoft Visual Studio/2022/Professional/VC/Tools/MSVC/14.34.31933/bin/Hostx64/x64/cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at CMakeLists.txt:4 (ADD_SUBDIRECTORY):
  The binary directory

    C:/Users/gkodinov/dev/fix-warnings-trunk/gogo/bld/SYSTEM

  is already used to build a source directory.  It cannot be used to build
  source directory

    C:/Users/gkodinov/dev/fix-warnings-trunk/gogo/b2

  Specify a unique binary directory name.


-- Configuring incomplete, errors occurred!
See also "C:/Users/gkodinov/dev/fix-warnings-trunk/gogo/bld/CMakeFiles/CMakeOutput.log".
2

Ok, I’ve got it. it’s a 3.25.1 feature.