fix build failure with glibc-2.28
    
    Reported at:
    https://lists.gnu.org/r/bug-gnulib/2018-03/msg00000.html
    
    ../lib/freadahead.c: In function 'freadahead':
    ../lib/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."
       ^~~~~
    ../lib/freadahead.c:99:1: warning: control reaches end of non-void function [-Wreturn-type]
     }
     ^
    make[2]: *** [Makefile:9144: lib/freadahead.o] Error 1
    make[2]: *** Waiting for unfinished jobs....
    ../lib/freadptr.c: In function 'freadptr':
    ../lib/freadptr.c:119:3: error: #error "Please port gnulib freadptr.c to your platform! Look at the definition of fflush, fread, getc, getc_unlocked on your system, then report this to bug-gnulib."
      #error "Please port gnulib freadptr.c to your platform! Look at the definition of fflush, fread, getc, getc_unlocked on your system, then report this to bug-gnulib."
       ^~~~~
    ../lib/freadptr.c:29:10: warning: unused variable 'size' [-Wunused-variable]
       size_t size;
              ^~~~
    ../lib/freadptr.c:121:1: warning: control reaches end of non-void function [-Wreturn-type]
     }
     ^