net/sfc: fix build with clang 3.4.2
net/sfc: fix build with clang 3.4.2

Old clang requires libatomic as well as gcc. Avoid compiler name and
version based checks. Add custom test for 16-byte atomic operations
to find out if libatomic is required to build.

Bugzilla ID: 760
Fixes: 96fd2bd69b58 ("net/sfc: support flow action count in transfer rules")

Signed-off-by: Andrew Rybchenko <4155e7bf8e0185127625dac6d84336299844982f@oktetlabs.ru>
Acked-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk