Skip target-import dependency verification procedure and test, if the host compiler does not support it.

Author: artemcm

Sources/Build/BuildOperation.swift

@@ -123,6 +123,10 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
     // Emit a warning if a target imports another target in this build
     // without specifying it as a dependency in the manifest
     private func verifyTargetImports(in description: BuildDescription) throws {
+        // Ensure the compiler supports the import-scan operation
+        guard SwiftTargetBuildDescription.checkSupportedFrontendFlags(flags: ["import-prescan"], fs: localFileSystem) else {
+            return
+        }
         for (target, commandLine) in description.swiftTargetScanArgs {
             do {
                 guard let dependencies = description.targetDependencyMap[target] else {

Tests/CommandsTests/BuildToolTests.swift

@@ -73,6 +73,9 @@ final class BuildToolTests: XCTestCase {
     }
 
     func testImportOfMissedDepWarning() throws {
+        #if swift(<5.5)
+        try XCTSkipIf(true, "skipping because host compiler doesn't support '-import-prescan'")
+        #endif
         fixture(name: "Miscellaneous/ImportOfMissingDependency") { path in
             let fullPath = resolveSymlinks(path)
             XCTAssertThrowsError(try build([], packagePath: fullPath)) { error in