Merge pull request #1 from yusuga/support_xcode12

Support for SPM in Xcode 12

Author: devxoul

Package.swift

@@ -4,6 +4,9 @@ import PackageDescription
 
 let package = Package(
   name: "WeakMapTable",
+  platforms: [
+    .macOS(.v10_11), .iOS(.v8), .tvOS(.v9), .watchOS(.v3)
+  ],
   products: [
     .library(name: "WeakMapTable", targets: ["WeakMapTable"]),
   ],