yacc.c, lalr1.cc: fix warnings about yynerrs

Most often yynerrs is not used.  Clang 15 now complains about such
variables.  Bison itself does not compile:

```
src/parse-gram.c:1797:9: error: variable 'gram_nerrs' set but not used [-Werror,-Wunused-but-set-variable]
    int yynerrs = 0;
        ^
src/parse-gram.c:79:25: note: expanded from macro 'yynerrs'
                        ^
1 error generated.
```

Reported by Nikita Popov.
Fixes https://github.com/akimd/bison/issues/89.

* data/skeletons/yacc.c (yynerrs): Flag with YY_ATTRIBUTE_UNUSED.
* data/skeletons/lalr1.cc (yynerrs_): Likewise.

THANKS

@@ -46,8 +46,8 @@ Csaba Raduly              [email protected]
 Dagobert Michelsen        [email protected]
 Daniel Frużyński          [email protected]
 Daniel Galloway           [email protected]
-Daniela Becker            [email protected]
 Daniel Hagerty            [email protected]
+Daniela Becker            [email protected]
 David Barto               [email protected]
 David J. MacKenzie        [email protected]
 David Kastrup             [email protected]
@@ -143,6 +143,7 @@ Nick Bowler               [email protected]
 Nicolas Bedon             [email protected]
 Nicolas Burrus            [email protected]
 Nicolas Tisserand         [email protected]
+Nikita Popov              [email protected]
 Nikki Valen               [email protected]
 Noah Friedman             [email protected]
 Odd Arild Olsen           [email protected]
@@ -170,7 +171,6 @@ Quoc Peyrot               [email protected]
 R Blake                   [email protected]
 Raja R Harinath           [email protected]
 Ralf Wildenhues           [email protected]
-Ryan                      [email protected]
 Rich Wilson               [email protected]
 Richard Stallman          [email protected]
 Rici Lake                 [email protected]
@@ -179,6 +179,7 @@ Rob Vermaas               [email protected]
 Robert Anisko             [email protected]
 Robert Yang               [email protected]
 Roland Levillain          [email protected]
+Ryan                      [email protected]
 Satya Kiran Popuri        [email protected]
 Sebastian Setzer          [email protected]
 Sebastien Fricker         [email protected]

data/skeletons/lalr1.cc

@@ -847,7 +847,7 @@ m4_if(b4_prefix, [yy], [],
     int yylen = 0;
 
     // Error handling.
-    int yynerrs_ = 0;
+    int yynerrs_ YY_ATTRIBUTE_UNUSED = 0;
     int yyerrstatus_ = 0;
 
     /// The lookahead symbol.

data/skeletons/yacc.c

@@ -256,7 +256,7 @@ int yynerrs;]])])
 m4_define([b4_declare_parser_state_variables],
 [b4_pure_if([[
     /* Number of syntax errors so far.  */
-    int yynerrs]m4_ifval([$1], [ = 0])[;
+    int yynerrs YY_ATTRIBUTE_UNUSED]m4_ifval([$1], [ = 0])[;
 ]])[
     yy_state_fast_t yystate]m4_ifval([$1], [ = 0])[;
     /* Number of tokens to shift before error messages enabled.  */