Add test for header include path lookup

Add a test case for the handling of cross-target header lookups which
would fail on Windows.

Author: millenomi

Fixtures/CFamilyTargets/CLibraryParentSearchPath/.gitignore

@@ -0,0 +1,9 @@
+.DS_Store
+/.build
+/Packages
+/*.xcodeproj
+xcuserdata/
+DerivedData/
+.swiftpm/config/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc

Fixtures/CFamilyTargets/CLibraryParentSearchPath/Package.swift

@@ -0,0 +1,29 @@
+// swift-tools-version: 5.5
+
+import PackageDescription
+
+let package = Package(
+    name: "CLibraryParentSearchPath",
+    products: [
+        .library(
+            name: "HeaderInclude",
+            targets: ["HeaderInclude"]),
+    ],
+    targets: [
+        .target(
+            name: "CHeaderInclude",
+            cSettings: [
+                /*
+                 This package tests path normalization; incorrect path normalization on certain OSes (especially Windows) can lead to relative paths like these not being correctly passed to the C compiler.
+                 */
+                .headerSearchPath("../Constants")
+            ]
+        ),
+        .target(
+            name: "HeaderInclude",
+            dependencies: [
+                "CHeaderInclude"
+            ]
+        )
+    ]
+)

Fixtures/CFamilyTargets/CLibraryParentSearchPath/Sources/CHeaderInclude/answers.c

@@ -0,0 +1,7 @@
+
+#include "answers.h"
+#include "Constants.h"
+
+int getAnswer(void) {
+    return kTheAnswer;
+}

Fixtures/CFamilyTargets/CLibraryParentSearchPath/Sources/CHeaderInclude/include/answers.h

@@ -0,0 +1,7 @@
+
+#ifndef answers_h
+#define answers_h
+
+extern int getAnswer(void);
+
+#endif /* answers_h */

Fixtures/CFamilyTargets/CLibraryParentSearchPath/Sources/Constants/Constants.h

@@ -0,0 +1,7 @@
+
+#ifndef Constants_h
+#define Constants_h
+
+static const int kTheAnswer = 42;
+
+#endif /* Constants_h */

Fixtures/CFamilyTargets/CLibraryParentSearchPath/Sources/HeaderInclude/FinalForm.swift

@@ -0,0 +1,6 @@
+
+import CHeaderInclude
+
+enum Answer {
+    var value: Int { Int(getAnswer()) }
+}

Tests/FunctionalTests/CFamilyTargetTests.swift

@@ -100,4 +100,11 @@ class CFamilyTargetTestCase: XCTestCase {
             XCTAssertSwiftTest(fixturePath)
         }
     }
+    
+    func testCanBuildRelativeHeaderSearchPaths() throws {
+        try fixture(name: "CFamilyTargets/CLibraryParentSearchPath") { fixturePath in
+            XCTAssertBuilds(fixturePath)
+            XCTAssertDirectoryContainsFile(dir: fixturePath.appending(components: ".build", UserToolchain.default.triple.platformBuildPathComponent(), "debug"), filename: "HeaderInclude.swiftmodule")
+        }
+    }
 }