programs, these two functions are
   automatically enabled if available.

4. Runtime functions not available on all platforms

   Some functions are not available on all systems.  One example of such a
   function is fstatat().  If possible, such functions will be replaced by
   wrappers, stubs or behavioral equivalent functions.  By default, such
   functions are disabled; enable them by setting #define HAVE_FUNC  1 (in
   environ.h).

   If you're building with the autoconf or cmake programs, these functi