[Xcodeproj] Add -DSWIFT_PACKAGE=1 to the C/C++ preprocessor defines

It was already set for swift, but needed to be added for C/C++/etc. to
match how this works for a command-line `swift build` invocation.

Also fixes a subtle bug in inherited resolution for build settings.

Author: benlangmuir

Sources/Xcodeproj/XcodeProjectModelSerialization.swift

@@ -441,8 +441,8 @@ fileprivate func combineBuildSettingsPropertyLists(
     // Iterate over the overlay values and apply them to the base.
     var resultDict = baseDict
     for (name, value) in overlayDict {
-        if let array = baseDict[name]?.array, array.first?.string == "$(inherited)" {
-            resultDict[name] = .array(array + (value.array ?? []))
+        if let array = baseDict[name]?.array, let overlayArray = value.array, overlayArray.first?.string == "$(inherited)" {
+            resultDict[name] = .array(array + overlayArray.dropFirst())
         } else {
             resultDict[name] = value
         }

Sources/Xcodeproj/pbxproj().swift

@@ -146,7 +146,8 @@ func xcodeProject(
     projectSettings.common.COMBINE_HIDPI_IMAGES = "YES"
 
     // Defined for regular `swift build` instantiations, so also should be defined here.
-    projectSettings.common.SWIFT_ACTIVE_COMPILATION_CONDITIONS += ["SWIFT_PACKAGE"]
+    projectSettings.common.SWIFT_ACTIVE_COMPILATION_CONDITIONS += ["$(inherited)", "SWIFT_PACKAGE"]
+    projectSettings.common.GCC_PREPROCESSOR_DEFINITIONS += ["$(inherited)", "SWIFT_PACKAGE=1"]
 
     // Opt out of headermaps.  The semantics of the build should be explicitly
     // defined by the project structure, so that we don't get any additional
@@ -161,10 +162,10 @@ func xcodeProject(
     projectSettings.debug.DEBUG_INFORMATION_FORMAT = "dwarf"
     projectSettings.debug.ENABLE_NS_ASSERTIONS = "YES"
     projectSettings.debug.GCC_OPTIMIZATION_LEVEL = "0"
-    projectSettings.debug.GCC_PREPROCESSOR_DEFINITIONS = ["DEBUG=1", "$(inherited)"]
+    projectSettings.debug.GCC_PREPROCESSOR_DEFINITIONS = ["$(inherited)", "DEBUG=1"]
     projectSettings.debug.ONLY_ACTIVE_ARCH = "YES"
     projectSettings.debug.SWIFT_OPTIMIZATION_LEVEL = "-Onone"
-    projectSettings.debug.SWIFT_ACTIVE_COMPILATION_CONDITIONS += ["SWIFT_PACKAGE", "DEBUG"]
+    projectSettings.debug.SWIFT_ACTIVE_COMPILATION_CONDITIONS += ["$(inherited)", "DEBUG"]
 
     // Add some release-specific settings.
     projectSettings.release.COPY_PHASE_STRIP = "YES"

Tests/XcodeprojTests/PackageGraphTests.swift

@@ -111,6 +111,7 @@ class PackageGraphTests: XCTestCase {
             XCTAssertEqual(project.buildSettings.debug.SWIFT_OPTIMIZATION_LEVEL, "-Onone")
             XCTAssertEqual(project.buildSettings.debug.SWIFT_ACTIVE_COMPILATION_CONDITIONS!, ["SWIFT_PACKAGE", "DEBUG"])
             XCTAssertEqual(project.buildSettings.common.SWIFT_ACTIVE_COMPILATION_CONDITIONS!, ["SWIFT_PACKAGE"])
+            XCTAssertEqual(project.buildSettings.common.GCC_PREPROCESSOR_DEFINITIONS, ["SWIFT_PACKAGE=1"])
 
             result.check(target: "Foo") { targetResult in
                 targetResult.check(productType: .framework)