[PATCH] acinclude.m4: fix -std=c23 build failure

[Rudi Heitbaum]

Add prototype to configure test function as c23 removes unprototyped functions.

gcc-15 switched to -std=c23 by default:

    https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=55e3bd376b2214e200fa76d12b67ff259b06c212

As a result `configure` fails with:
    conftest.c: In function 'f':
    conftest.c:12:48: error: too many arguments to function 'g'; expected 0, have 6
       12 | for(i=0;i<1;i++){if(e(got,got,9,d[i].n)==0)h();g(i,d[i].src,d[i].n,got,d[i].want,9);if(d[i].n)h();}}
          |                                                ^ ~
    conftest.c:7:6: note: declared here
        7 | void g(){}
          |      ^

Link: https://gmplib.org/list-archives/gmp-bugs/2024-November/005550.html
Signed-off-by: Rudi Heitbaum <rudi at heitbaum.com>
---
 acinclude.m4 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/acinclude.m4 b/acinclude.m4
index fddb5fb07a..2d41de6328 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -609,7 +609,7 @@ GMP_PROG_CC_WORKS_PART([$1], [long long reliability test 1],
 
 #if defined (__GNUC__) && ! defined (__cplusplus)
 typedef unsigned long long t1;typedef t1*t2;
-void g(){}
+void g(...){}
 void h(){}
 static __inline__ t1 e(t2 rp,t2 up,int n,t1 v0)
 {t1 c,x,r;int i;if(v0){c=1;for(i=1;i<n;i++){x=up[i];r=x+1;rp[i]=r;}}return c;}
-- 
2.43.0