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.