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 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_C_FLAGS cache entry. This occurs the first time a build tree is configured for language C. For any configuration run (including the first), the environment variable will be ignored if the CMAKE_C_FLAGS variable is already defined.