From Get docs
This is a CMake Environment Variable. Its initial value is taken from the calling process environment.
Default compilation flags to be used when compiling
CXX (C++) files. Will only be used by CMake on the first configuration to determine
CXX default compilation flags, after which the value for
CXXFLAGS is stored in the cache as CMAKE_CXX_FLAGS. For any configuration run ( including the first), the environment variable will be ignored if the CMAKE_CXX_FLAGS variable is defined.
See also CMAKE_CXX_FLAGS_INIT.
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