gatomic: disable GCC extension atomics for clang

clang defines the macro that we use to test for GCC's extension support
for C11 atomics, but doesn't define the extension in the same way.
Check for clang and disable the macros again if we find it.

https://bugzilla.gnome.org/show_bug.cgi?id=731513
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.