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
compilation flags, after which the value for
CXXFLAGS is stored in the cache
CMAKE_CXX_FLAGS. For any configuration run (
including the first), the environment variable will be ignored if
CMAKE_CXX_FLAGS variable is defined.