Compilers enable their extensions by default, and disabling them implicitly can lead to results which are surprising or non-obvious to debug. http://public.kitware.com/pipermail/cmake-developers/2014-May/010575.html http://thread.gmane.org/gmane.comp.programming.tools.cmake.devel/10214 https://www.mail-archive.com/cmake-developers@cmake.org/msg10116.html (Compiler feature extensions by default, 29 May 2014)
Link to it from the documentation of related properties, variables and commands. Extend the cmake-developer(7) documentation with notes on extending feature support for compilers.