don't use _Nonnull for clang 3.6 (#147) (#154)
don't use _Nonnull for clang 3.6 (#147) (#154)

Motivation:

Clang 3.6 doesn't know about `_Nonnull` which breaks compilation for our
C atomics.

Modifications:

Defined `_Nonnull` as `__attribute__((nonnull))` which seems to already
be available before clang 3.7.

Result:

NIO compiles with clang 3.6 as the default C compiler.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio