Fix build on Linux

Sources/SwiftyHolidays/CalculationPromise.swift

@@ -1,4 +1,9 @@
+#if canImport(Darwin)
 import class Dispatch.DispatchSemaphore
+#else
+@preconcurrency
+import class Dispatch.DispatchSemaphore
+#endif
 
 /// Represents a value that is currently being calculated.
 @usableFromInline

Sources/SwiftyHolidays/Gregorian/GregorianCalculator.swift

@@ -1,8 +1,4 @@
-#if canImport(Darwin)
 import Foundation
-#else
-@preconcurrency import Foundation // Calendar in Linux
-#endif
 
 /// Calculates holiday dates for the gregorian calendar.
 public struct GregorianCalculator: Calculator {

Tests/SwiftyHolidaysTests/Helpers.swift

@@ -0,0 +1,34 @@
+#if !canImport(Darwin) && swift(<5.9.1)
+import XCTest
+
+extension XCTestCase {
+    /// Wait on an array of expectations for up to the specified timeout, and optionally specify whether they
+    /// must be fulfilled in the given order. May return early based on fulfillment of the waited on expectations.
+    ///
+    /// - Parameter expectations: The expectations to wait on.
+    /// - Parameter timeout: The maximum total time duration to wait on all expectations.
+    /// - Parameter enforceOrder: Specifies whether the expectations must be fulfilled in the order
+    ///   they are specified in the `expectations` Array. Default is false.
+    /// - Parameter file: The file name to use in the error message if
+    ///   expectations are not fulfilled before the given timeout. Default is the file
+    ///   containing the call to this method. It is rare to provide this
+    ///   parameter when calling this method.
+    /// - Parameter line: The line number to use in the error message if the
+    ///   expectations are not fulfilled before the given timeout. Default is the line
+    ///   number of the call to this method in the calling file. It is rare to
+    ///   provide this parameter when calling this method.
+    ///
+    /// - SeeAlso: XCTWaiter
+    func fulfillment(of expectations: [XCTestExpectation], timeout: TimeInterval, enforceOrder: Bool = false) async {
+        await withCheckedContinuation { continuation in
+            // This function operates by blocking a background thread instead of one owned by libdispatch or by the
+            // Swift runtime (as used by Swift concurrency.) To ensure we use a thread owned by neither subsystem, use
+            // Foundation's Thread.detachNewThread(_:).
+            Thread.detachNewThread { [self] in
+                wait(for: expectations, timeout: timeout, enforceOrder: enforceOrder)
+                continuation.resume()
+            }
+        }
+    }
+}
+#endif