Hi,

On AIX 7.3.1, GNU sed fails to compile:

  CC       sed/sed-sed.o
../sed/sed.c:20:10: fatal error: getopt.h: No such file or directory
   20 | #include <getopt.h>
      |          ^~~~~~~~~~
compilation terminated.
make: 1254-004 The error code from the last command is 1.

Likewise on native Windows, with the MSVC compiler:

sed\sed.c(20): fatal error C1083: Cannot open include file: 'getopt.h': No such file or directory
make[2]: *** [Makefile:6034: sed/sed-sed.obj] Error 2

This patch fixes it, by using the Gnulib module 'getopt-gnu', as documented in
https://www.gnu.org/software/gnulib/manual/html_node/getopt_002eh.html