configure: Add configure-time check for GCC 4.0+ atomic built-ins.

configure: Add configure-time check for GCC 4.0+ atomic built-ins.

We found out earlier that GCC sometimes produces an error only at link time
for atomic built-ins that are not supported on a platform.  This actually
tries the link at configure time and should thus reliably detect whether
the atomic built-ins are really supported.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>