scripts/unifdef: avoid constexpr keyword

Starting with c23, 'constexpr' is a keyword in C like in C++ and cannot
be used as an identifier:

scripts/unifdef.c:206:25: error: 'constexpr' can only be used in variable declarations
206 | static bool constexpr; /* constant #if expression */
| ^
scripts/unifdef.c:880:13: error: expected identifier or '('
880 | constexpr = false;
| ^

Rename this instance to allow changing to C23 at some point in the future.

Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-By: Tony Finch <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/unifdef.c

···
 static int              delcount;		/* count of deleted lines */
 static unsigned         blankcount;		/* count of blank lines */
 static unsigned         blankmax;		/* maximum recent blankcount */
-static bool             constexpr;		/* constant #if expression */
+static bool             constexpression;	/* constant #if expression */
 static bool             zerosyms = true;	/* to format symdepth output */
 static bool             firstsym;		/* ditto */
 
···
 /*
  * Function for evaluating the innermost parts of expressions,
  * viz. !expr (expr) number defined(symbol) symbol
- * We reset the constexpr flag in the last two cases.
+ * We reset the constexpression flag in the last two cases.
  */
 static Linetype
 eval_unary(const struct ops *ops, int *valp, const char **cpp)
···
 		cp = skipcomment(cp);
 		if (defparen && *cp++ != ')')
 			return (LT_ERROR);
-		constexpr = false;
+		constexpression = false;
 	} else if (!endsym(*cp)) {
 		debug("eval%d symbol", ops - eval_ops);
 		sym = findsym(cp);
···
 			lt = *valp ? LT_TRUE : LT_FALSE;
 			cp = skipargs(cp);
 		}
-		constexpr = false;
+		constexpression = false;
 	} else {
 		debug("eval%d bad expr", ops - eval_ops);
 		return (LT_ERROR);
···
 	int val = 0;
 
 	debug("eval %s", *cpp);
-	constexpr = killconsts ? false : true;
+	constexpression = killconsts ? false : true;
 	ret = eval_table(eval_ops, &val, cpp);
 	debug("eval = %d", val);
-	return (constexpr ? LT_IF : ret == LT_ERROR ? LT_IF : ret);
+	return (constexpression ? LT_IF : ret == LT_ERROR ? LT_IF : ret);
 }
 
 /*