fix build failure with glibc-2.28
    
    Reported at:
    https://lists.gnu.org/r/bug-gnulib/2018-03/msg00000.html
    
    freadahead.c: In function ‘freadahead’:
    freadahead.c:97:3: error: #error "Please port gnulib freadahead.c to your platform! Look at the definition of fflush, fread, ungetc on your system, then report this to bug-gnulib."
      #error "Please port gnulib freadahead.c to your platform! Look at the definition of fflush, fread, ungetc on your system, then report this to bug-gnulib."
       ^~~~~