Exclude Foundation code on Linux

Package.swift

@@ -1,5 +1,10 @@
 import PackageDescription
 
-let package = Package(
-    name: "CryptoSwift"
-)
+let package = Package(name: "CryptoSwift")
+
+// Do not build Foundation part for non-darwin systems
+// where Swift Foundation is outdated on incomplete
+// see https://github.com/apple/swift/blob/master/lib/Basic/LangOptions.cpp#L26
+#if !os(OSX) && !os(tvOS) && !os(iOS) && !os(watchOS)
+package.exclude.append("Sources/CryptoSwift/Foundation")
+#endif