A few fixes for building SwiftPM for MacCatalyst (#6414)

Author: neonichu

Sources/Basics/Cancellator.swift

@@ -73,7 +73,7 @@ public final class Cancellator: Cancellable {
                 // Terminate all processes on receiving an interrupt signal.
                 try? self?.cancel(deadline: .now() + .seconds(30))
 
-#if os(macOS) || os(OpenBSD)
+#if os(macOS) || targetEnvironment(macCatalyst) || os(OpenBSD)
                 // Install the default signal handler.
                 var action = sigaction()
                 action.__sigaction_u.__sa_handler = SIG_DFL

Sources/swiftpm-xctest-helper/main.swift

@@ -138,7 +138,7 @@ do {
 
 #if os(Windows)
 import func ucrt.exit
-#else
+#elseif canImport(Glibc)
 import func Glibc.exit
 #endif
 print("Only macOS supported.")

Tests/PackageCollectionsSigningTests/Utilities.swift

@@ -62,10 +62,12 @@ struct TestCertificatePolicy: CertificatePolicy {
             #if os(macOS)
             self.verify(certChain: certChain, anchorCerts: self.anchorCerts, verifyDate: self.verifyDate,
                         callbackQueue: callbackQueue, callback: callback)
-            #else
+            #elseif os(Linux) || os(Windows) || os(Android)
             self.verify(certChain: certChain, anchorCerts: self.anchorCerts, verifyDate: self.verifyDate, httpClient: nil,
                         observabilityScope: ObservabilitySystem.NOOP,
                         callbackQueue: callbackQueue, callback: callback)
+            #else
+            throw InternalError("not implemented")
             #endif
         } catch {
             return callbackQueue.async { callback(.failure(error)) }