First SPM support attempt

Package.swift

@@ -0,0 +1,58 @@
+// swift-tools-version:5.3
+
+import PackageDescription
+
+let package = Package(
+    name: "zt",
+    products: [
+        .library(
+            name: "zt",
+            targets: ["zt"]),
+    ],
+    targets: [
+        .target(
+            name: "zt",
+            path: "",
+            exclude: [
+                "build",
+                "examples",
+                "ext/concurrentqueue",
+                "ext/lwip",
+                "ext/lwip-contrib",
+                "ext/ZeroTierOne/artwork",
+                "ext/ZeroTierOne/attic",
+                "ext/ZeroTierOne/controller",
+                "ext/ZeroTierOne/debian",
+                "ext/ZeroTierOne/ext",
+                "ext/ZeroTierOne/java",
+                "ext/ZeroTierOne/java",
+                "ext/ZeroTierOne/macui",
+                "ext/ZeroTierOne/osdep",
+                "ext/ZeroTierOne/rule-compiler",
+                "ext/ZeroTierOne/service",
+                "ext/ZeroTierOne/windows",
+                "ports",
+                "src/java",
+            ],
+            sources: [
+                "src/Controls.cpp",
+                "src/Events.cpp",
+                "src/NodeService.cpp",
+                "src/Sockets.cpp",
+                "src/VirtualTap.cpp",
+                
+                "ext/ZeroTierOne/node"
+            ],
+            cSettings: [
+                .headerSearchPath("src"),
+                .headerSearchPath("ext/concurrentqueue"),
+                .headerSearchPath("ext/lwip/src/include"),
+                .headerSearchPath("ext/lwip-contrib/ports/unix/port/include"),
+                .headerSearchPath("ext/ZeroTierOne/node"),
+                .headerSearchPath("ext/ZeroTierOne/osdep")
+            ],
+            cxxSettings: [
+                .unsafeFlags([ "-DOMIT_JSON_SUPPORT=1", "-std=c++11" ])
+            ])
+    ]
+)