Removed shared library variant.

Guard against multiple copies of Logger being linked.

Package.swift

@@ -8,17 +8,9 @@ let package = Package(
         .macOS(.v10_13), .iOS(.v12),
     ],
     products: [
-        .library(
-            name: "LoggerShared",
-            type: .dynamic,
-            targets: ["Logger"]),
         .library(
             name: "Logger",
             targets: ["Logger"]),
-        .library(
-            name: "LoggerKitShared",
-            type: .dynamic,
-            targets: ["LoggerKit"]),
         .library(
             name: "LoggerKit",
             targets: ["LoggerKit"]),

Sources/Logger/Channel.swift

@@ -52,7 +52,26 @@ public class Channel {
      though, which is why it's not a true singleton.
      */
 
-    public static let defaultManager = Manager()
+    public static let defaultManager = initDefaultManager()
+
+    /// Initialise the default log manager.
+    static func initDefaultManager() -> Manager {
+        #if ensureUniqueManager
+        /// We really do want there to only be a single instance of this, even if the logger library has mistakenly been
+        /// linked multiple times, so we store it in the thread dictionary for the main thread, and retrieve it from there if necessary
+        let dictionary = Thread.main.threadDictionary
+        if let manager = dictionary["Logger.Manager"] {
+            return unsafeBitCast(manager as AnyObject, to: Manager.self) // a normal cast might fail here if the code has been linked multiple times, since the class could be different (but identical)
+        }
+
+        let manager = Manager()
+        dictionary["Logger.Manager"] = manager
+        return manager
+
+        #else
+        return Manager()
+        #endif        
+    }
 
     /**
      Default subsystem if nothing else is specified.