Allowing caching build.

Package.swift

@@ -1,9 +1,16 @@
 import PackageDescription
-
+#if os(Linux)
+import Glibc
+#else
+import Darwin
+#endif
+var url = "https://github.com/PerfectlySoft/Perfect-CURL.git"
+if let urlenv = getenv("URL_PERFECT_CURL") {
+    url = String(cString: urlenv)
+}
 let package = Package(
     name: "PerfectSMTP",
     dependencies: [
-    .Package(url: "https://github.com/PerfectlySoft/Perfect-HTTP.git", majorVersion: 3),
-    .Package(url: "https://github.com/PerfectlySoft/Perfect-CURL.git", majorVersion: 3)
+    .Package(url: url, majorVersion: 3)
   ]
 )

Sources/PerfectSMTP/PerfectSMTP.swift

@@ -155,7 +155,7 @@ extension String {
   /// get the address info from a recipient, i.e, someone@somewhere -> @somewhere
   public var emailSuffix: String {
     get {
-      guard let at = self.characters.index(of: "@") else {
+      guard let at = self.index(of: "@") else {
         return self
       }//end at
       return self[at..<self.endIndex]
@@ -165,15 +165,15 @@ extension String {
   /// extract file name from a full path
   public var fileNameWithoutPath: String {
     get {
-      let segments = self.characters.split(separator: "/")
+      let segments = self.split(separator: "/")
       return String(segments[segments.count - 1])
     }//end get
   }//end fileNameWithoutPath
 
   /// extract file suffix from a file name
   public var suffix: String {
     get {
-      let segments = self.characters.split(separator: ".")
+      let segments = self.split(separator: ".")
       return String(segments[segments.count - 1])
     }//end get
   }//end suffix