use legacy atomics on gcc < 5
use legacy atomics on gcc < 5

mongodb/mongo-c-driver,beingmeta/mongo-c-driver,beingmeta/mongo-c-driver,mongodb/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,rcsanchez97/mongo-c-driver,mongodb/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,beingmeta/mongo-c-driver,beingmeta/mongo-c-driver,beingmeta/mongo-c-driver,jmikola/mongo-c-driver,mongodb/mongo-c-driver,beingmeta/mongo-c-driver,mongodb/mongo-c-driver,rcsanchez97/mongo-c-driver,mongodb/mongo-c-driver,rcsanchez97/mongo-c-driver,jmikola/mongo-c-driver,rcsanchez97/mongo-c-driver,mongodb/mongo-c-driver