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
For any configuration run (including the first), the environment variable
will be ignored if the
CMAKE_CXX_FLAGS variable is already defined.