Change main () to main (void) for C/C++

This patch changes "main ()" to "main (void)" for C/C++.
See: http://lists.gnu.org/archive/html/bug-autoconf/2014-01/msg00005.html

On my machine, before this patch, 3 tests were failing:

  38: tools.at:1329      autom4te cache locking
 218: autotest.at:1893   C unit tests
      ac_config_testdir at_tested autotest
 219: autotest.at:1948   C unit tests (EXEEXT)
      ac_config_testdir at_tested autotest

With this patch, 2 tests were failing:

 218: autotest.at:1893   C unit tests
      ac_config_testdir at_tested autotest
 219: autotest.at:1948   C unit tests (EXEEXT)
      ac_config_testdir at_tested autotest

(I suspect that 38 is unrelated.)

Signed-off-by: Vincent Lefevre <vincent@vinc17.net>
Copyright-paperwork-exempt: Yes