Add check for -latomic

Add check for -latomic

Later versions of gcc on some architectures push atomic functions
out into a separate atomic library; add a check to see when this
is required and add it to LIBS if need be.

Specifically the problem was observed on GCC 4.8.2 on powerpc
architecture for Ubuntu 14.04:

https://launchpadlibrarian.net/155235096/buildlog_ubuntu-trusty-powerpc.openvswitch_2.0.0-0ubuntu1_FAILEDTOBUILD.txt.gz

Signed-off-by: James Page <james.page@ubuntu.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>