export CXX=/dev/null 
 bjori@TaylorSwift  ~/Sources/libbson/cbuild   master  cmake ..
-- The C compiler identification is GNU 6.3.0
-- The CXX compiler identification is unknown
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /dev/null
-- Check for working CXX compiler: /dev/null -- broken
CMake Error at /usr/share/cmake-3.7/Modules/CMakeTestCXXCompiler.cmake:44 (message):
  The C++ compiler "/dev/null" is not able to compile a simple test program.