Support a file name argument to %defines. Deprecate `=' in

Support a file name argument to %defines.  Deprecate `=' in
%file-prefix, %name-prefix, and %output.  Discussed at
<http://lists.gnu.org/archive/html/help-bison/2006-09/msg00001.html>.
* NEWS (2.3a+): Mention.
* doc/bison.texinfo (Decl Summary, Bison Symbols): Add entry for new
form of %defines, and remove `=' from entries for %file-prefix,
%name-prefix, and %output.
* src/parse-gram.y (prologue_declaration): Implement.
* tests/calc.at (Simple LALR Calculator, Simple GLR Calculator, Simple
LALR1 C++ Calculator, Simple GLR C++ Calculator): Remove the `=' from
all but one occurrence of %name-prefix.
* tests/headers.at (export YYLTYPE): Remove the `=' from %name-prefix.
* tests/local.at (_AT_BISON_OPTION_PUSHDEFS): Likewise.
* tests/output.at (AT_CHECK_OUTPUT): Remove the `=' from all but one
occurrence of each of %file-prefix and %output.  Add check for %defines
with argument.
* tests/reduce.at (Useless Terminals, Useless Nonterminals,
Useless Rules, Reduced Automaton, Underivable Rules, Empty Language):
Remove the `=' from %output.