[PATCH] AC_C_CONST: support -Werror=unused-but-set-variable with GCC 16

Needed with gcc r16-2258-g0eac9cfee8cb0b21d, which
extends -Wunused-but-set-* warnings:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677#c25

Signed-off-by: Vincent Lefevre <[email protected]>
---
 lib/autoconf/c.m4 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 4cdb2995..3705d028 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -2141,11 +2141,13 @@ AC_DEFUN([AC_C_CONST],
     int x[] = {25, 17};
     const int *foo = &x[0];
     ++foo;
+    if (!foo) return 0;
   }
   { /* Sun SC1.0 ANSI compiler rejects this -- but not the above. */
     typedef const int *iptr;
     iptr p = 0;
     ++p;
+    if (!p) return 0;
   }
   { /* IBM XL C 1.02.0.0 rejects this sort of thing, saying
        "k.c", line 2.27: 1506-025 (S) Operand must be a modifiable lvalue. */
-- 
2.50.1