[pkg-config] Lazily compute current platform

Author: aciidgh

Sources/Build/misc.swift

@@ -189,7 +189,7 @@ extension SystemPackageProvider {
     }
 
     var isAvailable: Bool {
-        guard let platform = Platform.currentPlatform() else { return false }
+        guard let platform = Platform.currentPlatform else { return false }
         switch self {
         case .Brew(_):
             if case .Darwin = platform  {

Sources/Utility/Platform.swift

@@ -18,7 +18,9 @@ public enum Platform {
         case Debian
     }
 
-    public static func currentPlatform() -> Platform? {
+    // Lazily return current platform.
+    public static var currentPlatform = Platform.findCurrentPlatform()
+    private static func findCurrentPlatform() -> Platform? {
         guard let uname = try? popen(["uname"]).chomp().lowercased() else { return nil }
         switch uname {
         case "darwin":