Add Swift Package Manager and Linux support

.gitignore

@@ -1,3 +1,4 @@
 .DS_Store
+.build/
 xcuserdata/
-project.xcworkspace/
+project.xcworkspace/

Package.swift

@@ -0,0 +1,22 @@
+// swift-tools-version:4.1
+
+import PackageDescription
+
+let package = Package(
+    name: "VectorMath",
+    products: [
+        .library(
+            name: "VectorMath",
+            targets: ["VectorMath"]),
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "VectorMath",
+            dependencies: [],
+            path: "VectorMath"),
+        .testTarget(
+            name: "VectorMathTests",
+            dependencies: ["VectorMath"]),
+    ]
+)

Tests/VectorMathTests/VectorMathTests.swift

@@ -9,6 +9,10 @@
 import QuartzCore
 import XCTest
 
+#if SWIFT_PACKAGE
+@testable import VectorMath
+#endif
+
 class Vector2Tests: XCTestCase {
 
     func testRotatedBy() {

VectorMath/VectorMath+Quartz.swift

@@ -31,6 +31,8 @@
 //  3. This notice may not be removed or altered from any source distribution.
 //
 
+#if canImport(QuartzCore)
+
 import QuartzCore
 
 // MARK: SceneKit extensions
@@ -110,3 +112,5 @@ public extension Matrix4 {
         )
     }
 }
+
+#endif

VectorMath/VectorMath+SceneKit.swift

@@ -31,6 +31,8 @@
 //  3. This notice may not be removed or altered from any source distribution.
 //
 
+#if canImport(SceneKit)
+
 #if os(iOS) || os(tvOS)
 typealias SCNFloat = Float
 #else
@@ -108,3 +110,5 @@ public extension Quaternion {
         self.init(x: Scalar(q.x), y: Scalar(q.y), z: Scalar(q.z), w: Scalar(q.w))
     }
 }
+
+#endif