always capture exceptions by const references

For #199.

* tests/sanity/style.test: Test this.
* bin/ltlcross.cc, spot/parseaut/parseaut.yy,
tests/ltsmin/modelcheck.cc: Fix it.

bin/ltlcross.cc

@@ -1403,7 +1403,7 @@ namespace
                       sm = new
                         spot::scc_info(p, spot::scc_info_options::TRACK_STATES);
                     }
-                  catch (std::bad_alloc&)
+                  catch (const std::bad_alloc&)
                     {
                       std::cerr << ("warning: not enough memory to build "
                                     "product of P") << i << " with state-space";
@@ -1440,7 +1440,7 @@ namespace
                           spot::scc_info(p,
                                          spot::scc_info_options::TRACK_STATES);
                       }
-                    catch (std::bad_alloc&)
+                    catch (const std::bad_alloc&)
                       {
                         std::cerr << ("warning: not enough memory to build "
                                       "product of N")

spot/parseaut/parseaut.yy

 /* -*- coding: utf-8 -*-
-** Copyright (C) 2014, 2015, 2016, 2017 Laboratoire de Recherche et
-** Développement de l'Epita (LRDE).
+** Copyright (C) 2014-2018 Laboratoire de Recherche et Développement
+** de l'Epita (LRDE).
 **
 ** This file is part of Spot, a model checking library.
 **
@@ -2576,7 +2576,7 @@ namespace spot
 	automaton_stream_parser p(filename, localopts);
 	pa = p.parse(dict, env);
       }
-    catch (std::runtime_error& e)
+    catch (const std::runtime_error& e)
       {
 	if (opts.raise_errors)
 	  throw;

tests/ltsmin/modelcheck.cc

@@ -222,7 +222,7 @@ checked_main(int argc, char **argv)
           model = spot::ltsmin_model::load(argv[1]).kripke(&ap, dict, deadf,
                                                            compress_states);
         }
-      catch (std::runtime_error& e)
+      catch (const std::runtime_error& e)
         {
           std::cerr << e.what() << '\n';
         }

tests/sanity/style.test

@@ -287,6 +287,10 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
 	 diag 'No "if (x)" required before "delete x;".'
       fi
 
+      # we want catch (const reftype&) or catch (...)
+      $GREP 'catch *([^.]' $tmp | $GREP -v 'const.*\&' &&
+	diag 'Always capture exceptions by const reference.'
+
       case $file in
 	*.hh | *.hxx)
 	  if e$GREP '(<<|>>)' $tmp >/dev/null; then