Suppress unused-but-set-variable warning in parsers

This is very ugly: Bison provides a yynerrs variable, which is
usually not actually used, but also not annotated with
YY_MAYBE_UNUSED. Suppress this warning by adding a (void)yynerrs
into the top-level reduction action. The alternative would be to
disable the warning for these generated files.

Zend/zend_ini_parser.y

@@ -308,7 +308,7 @@ static void zval_ini_dtor(zval *zv)
 
 statement_list:
 		statement_list statement
-	|	%empty
+	|	%empty { (void) ini_nerrs; }
 ;
 
 statement:

Zend/zend_language_parser.y

@@ -292,7 +292,7 @@ static YYSIZE_T zend_yytnamerr(char*, const char*);
 %% /* Rules */
 
 start:
-	top_statement_list	{ CG(ast) = $1; }
+	top_statement_list	{ CG(ast) = $1; (void) zendnerrs; }
 ;
 
 reserved_non_modifiers:

ext/json/json_parser.y

@@ -80,6 +80,7 @@ start:
 			{
 				ZVAL_COPY_VALUE(&$$, &$1);
 				ZVAL_COPY_VALUE(parser->return_value, &$1);
+				(void) php_json_yynerrs;
 				YYACCEPT;
 			}
 ;

sapi/phpdbg/phpdbg_parser.y

@@ -66,7 +66,7 @@ ZEND_EXTERN_MODULE_GLOBALS(phpdbg)
 input
 	: command { $$ = $1; }
 	| input T_SEPARATOR command { phpdbg_stack_separate($1.top); $$ = $3; }
-	| %empty
+	| %empty { (void) phpdbg_nerrs; }
 	;
 
 command