[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.

Author: benlangmuir

Sources/Xcodeproj/pbxproj().swift

@@ -147,6 +147,7 @@ func xcodeProject(
 
     // Defined for regular `swift build` instantiations, so also should be defined here.
     projectSettings.common.SWIFT_ACTIVE_COMPILATION_CONDITIONS += ["SWIFT_PACKAGE"]
+    projectSettings.common.GCC_PREPROCESSOR_DEFINITIONS += ["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

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)