acinclude.m4: fix configure tests broken with Clang 16 (implicit function declarations)

Clang 16 will make -Wimplicit-int and -Wimplicit-function-declaration
errors by default which can, in addition to other things, lead to
some configure tests silently failing/returning the wrong result.

For more information, see LWN.net [0] or LLVM's Discourse [1], gentoo-dev@ [2],
or the (new) c-std-porting mailing list [3].

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
[2] https://archives.gentoo.org/gentoo-dev/message/dd9f2d3082b8b6f8dfbccb0639e6e240
[3] hosted at lists.linux.dev.

Signed-off-by: Sam James <sam@gentoo.org>

acinclude.m4

@@ -904,7 +904,8 @@ AC_DEFUN([AC_W3M_SIGSETJMP],
 [AC_SUBST(HAVE_SIGSETJMP)
 AC_MSG_CHECKING(for sigsetjmp)
 AC_TRY_COMPILE(
-[#include <setjmp.h>],
+[#include <setjmp.h>
+ #include <stdlib.h>],
 [ jmp_buf env;
    if (sigsetjmp(env, 1) != 0) { exit(0); } siglongjmp(env, 1);],
 [have_sigsetjmp="yes"; AC_DEFINE(HAVE_SIGSETJMP)],