Implement lookahead correction (LAC) for the C++ skeleton. LAC is a
mechanism to make sure that we report the correct list of expected
tokens if a syntax error occurs. So far, LAC was only supported for
the C skeleton "yacc.c".
* data/skeletons/lalr1.cc: Add LAC support.
* doc/bison.texi: Update.
* data/skeletons/yacc.c (yysyntax_error): Change the nesting of `m4`
conditions slightly to make it more readable.
The generated C code stays unchanged.