We cannot use dylib's on iOS in the App Store. So we need to swtich to
using a static library.
This means we cannot use `LoadLibrary` on iOS. So lets switch over to
using `DllImport`. We can do this now since we control the name of the
library we ship. It is consistently `openal` (or `libopenal.*`) on all
platforms.

Version 1.24.3.2 of OpenAL has the static libs for iOS.