configure fixes for gcc-14

[paparodeo]

fixes build with gcc-14. the on_exit check fails due to

test.c: In function ‘main’:
test.c:3:26: error: passing argument 1 of ‘on_exit’ from incompatible pointer type [-Wincompatible-pointer-types]
3 | int main(void) { on_exit(handler, (void )0); return 0; }
| ^~~~~~~
| |
| void ()(void)
In file included from test.c:1:
/usr/include/stdlib.h:749:28: note: expected ‘void ()(int, void )’ but argument is of type ‘void ()(void)’
749 | extern int on_exit (void (__func) (int __status, void *__arg), void *__arg)

but given that util.c will fallback to atexit that seems fine.