configure.ac: fix check for futex and eventfd

configure.ac: fix check for futex and eventfd

We were using AC_LANG_PROGRAM to build a program to test for our ability
to call syscall (__NR_futex, ...);.  This macro adds "main () { ... }"
around the provided code segment automatically.  The provided code
segment provided its own main() function, however.

The result looked something like:

int main (void) {
  int main (void) {
    ...
  }
}

which worked on GCC, but not on clang.

Let's fix that.  Let's fix the same mistake copied over for eventfd()
detection while we're at it.