forbid the use of std::endl on std::cerr

std::cerr will flush after each operator<< by default, so it's simpler
to use \n instead of std::endl, especially if we can merge \n into the
previous string.  Ideally we should prefer \n for std::cout as well,
but there are reasonable cases where we want to call std::endl there,
so it's hard to enforce.

* tests/sanity/style.test: Diagnose occurrences of cerr.*<<.*endl.
* bin/autcross.cc, bin/autfilt.cc, bin/ltlcross.cc, bin/ltlsynt.cc,
spot/tl/formula.cc, spot/twa/bdddict.cc, tests/core/checkpsl.cc,
tests/core/checkta.cc, tests/core/consterm.cc, tests/core/emptchk.cc,
tests/core/equalsf.cc, tests/core/ikwiad.cc, tests/core/kind.cc,
tests/core/length.cc, tests/core/ltlrel.cc, tests/core/parity.cc,
tests/core/randtgba.cc, tests/core/reduc.cc, tests/core/syntimpl.cc,
tests/ltsmin/modelcheck.cc: Fix them.

tests/sanity/style.test

@@ -1,6 +1,6 @@
 #! /bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2009-2018 Laboratoire de Recherche et Développement de
+# Copyright (C) 2009-2019 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 # Copyright (C) 2004, 2005 Laboratoire d'Informatique de Paris 6
 # (LIP6), département Systèmes Répartis Coopératifs (SRC), Université
@@ -67,7 +67,6 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
                  -a -not -path '*.dir/*' \
                  -a -type f -a -print |
   while read file; do
-
     if $GREP 'GNU Bison' "$file" >/dev/null ||
       $GREP 'generated by flex' "$file" >/dev/null ; then
       continue
@@ -266,6 +265,9 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
       e$GREP '(->|[.])size\(\) [=!]= 0|![a-zA-Z0-9_]*(->|[.])size\(\)|(if |while |assert)\([a-zA-Z0-9_]*(->|[.])size\(\)\)' $tmp &&
 	diag 'Prefer empty() to check emptiness.'
 
+      $GREP -E 'std::cerr.*<<.*endl' $tmp &&
+        diag 'std::cerr has unitbuf set; use \\n instead of endl'
+
       e$GREP 'assert\((0|!".*")\)' $tmp &&
         diag 'Prefer SPOT_UNREACHABLE or SPOT_UNIMPLEMENTED.'