devel/m4 fails with glibc 2.28

["Sijmen J. Mulder" <ik%sjmulder.nl@localhost>]

Hi all,

On an up-to-date Ubuntu x86-64 after a fresh bootstrap, devel/m4
(1.4.18) fails to build:

  freadahead.c:92:3: 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.

The failing check:

  #if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 \
      /* GNU libc, BeOS, Haiku, Linux libc5 */

Others are reporting the same issue, pointing at glibc 2.28:

  https://lists.gnu.org/r/bug-gnulib/2018-03/msg00000.html
  https://bugs.archlinux.org/task/59562
  http://lists.gnu.org/archive/html/bug-m4/2018-11/msg00001.html

A reply to the last thread includes a patch:

  https://lists.gnu.org/r/bug-gnulib/2018-03/msg00002.html

Leaving this here because I don't know enough about M4 or the libc
semantics to tell if the patch is any good (and I haven't tried it
either).

Sijmen