configure: Include <stdlib.h> for system, exit

Otherwise the conf.sanetest check will always fail with compilers
that do not support implicit function declarations.

acinclude.m4

@@ -484,6 +484,7 @@ dnl
       AC_TRY_RUN([
 #include <sane/sane.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int 
 main ()

aclocal.m4

@@ -494,6 +494,7 @@ dnl
       AC_TRY_RUN([
 #include <sane/sane.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int 
 main ()

configure

@@ -7565,6 +7565,7 @@ cat >>conftest.$ac_ext <<_ACEOF
 
 #include <sane/sane.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 int
 main ()