A program

    #include <langinfo.h>
    #include <stdio.h>

    int
    main (void)
    {
      printf ("%s\n", nl_langinfo (DECIMAL_POINT));
      return 0;
    }

compiled with gcc foo.c gives an error

    foo.c:7: error: 'DECIMAL_POINT' undeclared (first use in this function)

I see that it's necessary have #define _GNU_SOURCE to get
DECIMAL_POINT defined from langinfo.h.  I think the node "The Elegant
and Fast Way" in the manual describing DECIMAL_POINT should note that
or whatever feature test macro is necessary (assuming this is
intentional).