Add defensive @available guards to prevent accidental compilation on …

…unsupported platforms.

Sources/ApolloCodegenLib/ApolloCLI.swift

@@ -1,6 +1,7 @@
 import Foundation
 
 /// Wrapper for calling the bundled node-based Apollo CLI.
+@available(OSX, message: "Only available on macOS")
 public struct ApolloCLI {
 
   /// Creates an instance of `ApolloCLI`, downloading and extracting if needed

Sources/ApolloCodegenLib/ApolloCodegen.swift

@@ -1,6 +1,7 @@
 import Foundation
 
 /// A class to facilitate running code generation
+@available(OSX, message: "Only available on macOS")
 public class ApolloCodegen {
 
   /// Errors which can happen with code generation

Sources/ApolloCodegenLib/ApolloSchemaDownloader.swift

@@ -1,6 +1,7 @@
 import Foundation
 
 /// A wrapper to facilitate downloading a schema with the Apollo node CLI
+@available(OSX, message: "Only available on macOS")
 public struct ApolloSchemaDownloader {
 
   /// Runs code generation from the given folder with the passed-in options

Sources/ApolloCodegenLib/Basher.swift

@@ -1,6 +1,7 @@
 import Foundation
 
 /// Bash command runner
+@available(OSX, message: "Only available on macOS")
 public struct Basher {
 
   public enum BashError: Error, LocalizedError {

Sources/ApolloCodegenLib/CLIDownloader.swift

@@ -1,6 +1,7 @@
 import Foundation
 
 /// Helper for downloading the CLI Zip file so we don't have to include it in the repo.
+@available(OSX, message: "Only available on macOS")
 struct CLIDownloader {
 
   enum CLIDownloaderError: Error, LocalizedError {

Sources/ApolloCodegenLib/CLIExtractor.swift

@@ -1,6 +1,7 @@
 import Foundation
 
 /// Helper for extracting and validating the node-based Apollo CLI from a zip.
+@available(OSX, message: "Only available on macOS")
 struct CLIExtractor {
 
   // MARK: - Extracting the binary