tests/test-gendoc-zh_TW.t

fuzz: compile xdiff.cc with -std=c++17

Otherwise the build would fail as follows:

  In file included from xdiff.cc:13:
  ./fuzzutil.h:23:10: fatal error: 'third_party/absl/types/optional.h' file not
  found
  #include "third_party/absl/types/optional.h"

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors