Add a Swift Package Manager manifest

This one only builds libolm, not yet the Xcode stuff.

Package.swift

@@ -0,0 +1,29 @@
+// swift-tools-version:5.2
+
+import PackageDescription
+
+let version = ( major: 3, minor: 1, patch: 4 )
+
+let package = Package(
+    name: "Olm",
+    products: [
+        .library(name: "libolm", targets: ["libolm"])
+    ],
+    targets: [
+        .target(
+            name: "libolm",
+            path: ".",
+            sources: [ "src" ],
+            publicHeadersPath: "include",
+            cSettings: [
+              .define("OLMLIB_VERSION_MAJOR", to: String(version.major)),
+              .define("OLMLIB_VERSION_MINOR", to: String(version.minor)),
+              .define("OLMLIB_VERSION_PATCH", to: String(version.patch)),
+              .headerSearchPath("lib"),
+              .unsafeFlags([ "-Wall", "-Werror" ])
+            ]
+        )
+    ],
+    cLanguageStandard: .c99,
+    cxxLanguageStandard: .cxx11
+)