build: define gitsha for swift package builds

src/private/mx/core/GitShaDefault.cpp

@@ -0,0 +1,21 @@
+// MusicXML Class Library
+// Copyright (c) by Matthew James Briggs
+// Distributed under the MIT License
+
+// The Swift package build's definition of mx::core::gitSha. CMake builds generate the real
+// value at build time (cmake/GitSha.cmake renders Version.cpp.in into the build tree) and list
+// mx_core's sources explicitly, so they never compile this file. The Swift package build globs
+// src/ and has no code-generation step, so without this fallback the Mx product carries an
+// undefined symbol that surfaces only when a consumer links an executable. "unknown" is the
+// value Version.h already documents for builds outside the git/CMake machinery.
+//
+// Do not add this file to any CMake source list: the linker would see two definitions.
+
+#include "mx/core/Version.h"
+
+namespace mx::core
+{
+
+const char *const gitSha = "unknown";
+
+} // namespace mx::core