openal: fix atomic handling

openal can uses __atomic_*() intrinsics when available, or fall-back
on __sync_*() built-ins. This commit adjusts the package to take into
account for those dependencies, and makes sure we link against
libatomic when gcc >= 4.8 so that the __atomic_*() intrinsics can be
used on all architectures.

Fixes:

  http://autobuild.buildroot.org/results/314fb8da3ca87984e9240d67ff233e2f999ae89e/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>