New in version 3.1.
Default value for CXX_EXTENSIONS property of targets.
This variable is used to initialize the CXX_EXTENSIONS property on all targets. See that target property for additional information.
See the cmake-compile-features(7) manual for information on compile features and a list of supported compilers.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.19/variable/CMAKE_CXX_EXTENSIONS.html