New in version 3.7.

Value used to initialize the CMAKE_<LANG>_FLAGS cache entry the first time a build tree is configured for language <LANG>. This variable is meant to be set by a toolchain file. CMake may prepend or append content to the value based on the environment and target platform. For example, the contents of a xxxFLAGS environment variable will be prepended, where xxx will be language-specific but not necessarily the same as <LANG> (e.g. CXXFLAGS for CXX, FFLAGS for Fortran, and so on). This value is a command-line string fragment. Therefore, multiple options should be separated by spaces, and options with spaces should be quoted.

See also the configuration-specific CMAKE_<LANG>_FLAGS_<CONFIG>_INIT variable.