Avoid calling the undeclared exit functions, to avoid build problems
with compilers that no longer support implicit function declarations.

diff --git a/configure.ac b/configure.ac
index c3ebffb887f6bb82..1208a0eefa675766 100644
--- a/configure.ac
+++ b/configure.ac
@@ -663,6 +663,7 @@ AC_CACHE_CHECK([for TIOCLINUX],tucnak_cv_HAVE_TIOCLINUX,[
 AC_TRY_RUN([
 #define AUTOCONF_TEST 1
 #include <sys/ioctl.h>
+#include <stdlib.h>
 #include "confdefs.h"
 int cmp(const struct dirent **a, const struct dirent **b){return 0;}
 int main(){
@@ -686,7 +687,6 @@ AC_TRY_RUN([
 #include "confdefs.h"
 int main(){
     socklen_t tmp;
-    exit(0);
 }],
 tucnak_cv_HAVE_SOCKLEN_T=yes,tucnak_cv_HAVE_SOCKLEN_T=no,tucnak_cv_HAVE_SOCKLEN_T=cross)])