Minor fixups to get 'make maintainer-check' to work.

* configure.ac: Don't use -Wnested-externs, as it's incompatible
with the new verify.h implementation.
* data/c.m4 (b4_yy_symbol_print_generate): YYUSE (yyoutput).
* data/glr.c (YYUSE): Depend on __GNUC__ as well.
* data/yacc.c (YYUSE): Likewise.
* data/lalr1.cc (yysyntax_error_): YYUSE (yystate).
* lib/subpipe.c (end_of_output_subpipe): The args are unused.
* src/parse-gram.y (declaration): Don't pass a string constant
to a function that expects char *, since GCC might complain
about the constant value.
* src/reader.c (symbol_typed_p): Add parens to pacify GCC.
* tests/cxx-type.at (_AT_TEST_GLR_CXXTYPES): #undef yylloc and yylval
before #defining them.
* tests/glr-regression.at
(Incorrectly initialized location for empty right-hand side in GLR):
In yyerror, use the msg arg.
(Corrupted semantic options if user action cuts parse):
(Incorrect lookahead during deterministic GLR):
(Incorrect lookahead during nondeterministic GLR):
Don't name a local var 'index'; it shadows string.h's 'index'.

data/lalr1.cc

@@ -829,6 +829,7 @@ b4_error_verbose_if([, yytoken])[));
 b4_error_verbose_if([, int tok])[)
   {
     std::string res;
+    YYUSE (yystate);
 #if YYERROR_VERBOSE
     int yyn = yypact_[yystate];
     if (yypact_ninf_ < yyn && yyn < yylast_)