This is a CMake Environment Variable. Its initial value is taken from the calling process environment.

Add default compilation flags to be used when compiling CXX (C++) files.

CMake uses this environment variable value, in combination with its own builtin default flags for the toolchain, to initialize and store the CMAKE_CXX_FLAGS cache entry. This occurs the first time a build tree is configured for language CXX. For any configuration run (including the first), the environment variable will be ignored if the CMAKE_CXX_FLAGS variable is already defined.