Configure targets for lz4, mbedTLS and OpenVPN3

Package.swift

@@ -0,0 +1,63 @@
+// swift-tools-version:5.1
+
+import PackageDescription
+
+let package = Package(
+    name: "OpenVPNAdapter",
+    platforms: [
+        .iOS("9.0"),
+        .macOS("10.11"),
+    ],
+    products: [
+        .library(name: "OpenVPNAdapter", type: .static, targets: ["OpenVPNAdapter"]),
+    ],
+    targets: [
+        .target(
+            name: "OpenVPNAdapter",
+            dependencies: [
+                .target(name: "OpenVPN3")
+            ],
+            cxxSettings: [
+                .headerSearchPath("../OpenVPN3"),
+            ]
+        ),
+        .target(
+            name: "LZ4",
+            sources: ["lib"],
+            cSettings: [
+                .define("XXH_NAMESPACE", to: "LZ4_")
+            ]
+        ),
+        .target(
+            name: "mbedTLS", 
+            sources: ["library"],
+            cSettings: [
+                .define("MBEDTLS_MD4_C"),
+                .define("MBEDTLS_RELAXED_X509_DATE"),
+                .define("_FILE_OFFSET_BITS", to: "64"),
+            ]
+        ),
+        .target(
+            name: "OpenVPN3",
+            dependencies: [
+                .target(name: "LZ4"),
+                .target(name: "mbedTLS")
+            ],
+            sources: ["library"],
+            cxxSettings: [
+                .headerSearchPath("."),
+                .headerSearchPath("../ASIO/asio/include"),
+                .define("ASIO_STANDALONE"),
+                .define("ASIO_NO_DEPRECATED"),
+                .define("ASIO_HAS_STD_STRING_VIEW"),
+                .define("USE_ASIO"),
+                .define("USE_ASIO_THREADLOCAL"),
+                .define("HAVE_LZ4"),
+                .define("USE_MBEDTLS"),
+                .define("OPENVPN_FORCE_TUN_NULL"),
+                .define("USE_TUN_BUILDER")
+            ]
+        )
+    ],
+    cxxLanguageStandard: .gnucxx14
+)