Set EXECUTABLE_PREFIX for shared libraries

- use a build setting to set the missing "lib" prefix on platforms
  that should have one (all but windows)

Author: daveinglis

Sources/SwiftBuildSupport/PackagePIFBuilder+Helpers.swift

@@ -1032,6 +1032,11 @@ extension ProjectModel.BuildSettings {
         self[.PRODUCT_BUNDLE_IDENTIFIER] = "\(packageIdentity).\(productName)".spm_mangledToBundleIdentifier()
         self[.SWIFT_PACKAGE_NAME] = packageName ?? nil
 
+        //This should really be swift-build defaults set in the .xcspec files, but changing that requires some extensive testing to ensure xcode projects are not effected.
+        // So for now lets just force it here.
+        self[.EXECUTABLE_PREFIX] = "lib"
+        self[.EXECUTABLE_PREFIX, Platform.windows] = ""
+
         if !createDylibForDynamicProducts {
             self[.GENERATE_INFOPLIST_FILE] = "YES"
             // If the built framework is named same as one of the target in the package,