For associating token numbers with token names for "yacc.c", don't use

For associating token numbers with token names for "yacc.c", don't use
#define statements unless `--yacc' is specified; always use enum
yytokentype.  Most important discussions start at:
<http://lists.gnu.org/archive/html/bison-patches/2005-09/msg00053.html>,
<http://lists.gnu.org/archive/html/bison-patches/2005-12/msg00052.html>,
and
<http://lists.gnu.org/archive/html/bison-patches/2006-06/msg00043.html>.
* NEWS (2.3+): Mention.
* data/c.m4 (b4_yacc_if): New.
(b4_token_enums_defines): Use b4_yacc_if to decide whether to add the
token #define's.
* doc/bison.texinfo (Bison Options): Describe the effect of `--yacc'
on token name definitions.
* src/getargs.c (usage): Capitalize `Yacc' in English.
* src/output.c (prepare): Define b4_yacc_flag.
* tests/regression.at (Early token definitions): Test that tokens names
are defined before the pre-prologue not just before the post-prologue.
Remove this test case and copy to...
(Early token definitions with --yacc): ... this to test #define's.
(Early token definitions without --yacc): ... and this to test enums.