configure: Fix compiler warnings in config.log (old-style function definition)

warning: function declaration isn’t a prototype
In function ‘foo’:
warning: old-style function definition

The function name was changed, too, to avoid an additional warning.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1274,11 +1274,11 @@ if test "$nptl" != "no" ; then
   cat > $TMPC <<EOF
 #include <sched.h>
 #include <linux/futex.h>
-void foo()
-{
+int main(void) {
 #if !defined(CLONE_SETTLS) || !defined(FUTEX_WAIT)
 #error bork
 #endif
+  return 0;
 }
 EOF