Update Versions.cpp

Check the major clang version before attempting to use `__has_embed` or `__clang_major__`.

Author: grynspan

Sources/_TestingInternals/Versions.cpp

@@ -18,7 +18,8 @@ const char *swt_getTestingLibraryVersion(void) {
 #if defined(SWT_TESTING_LIBRARY_VERSION)
   // The current environment explicitly specifies a version string to return.
   return SWT_TESTING_LIBRARY_VERSION;
-#elif __has_embed("../../VERSION.txt")
+#elif __clang_major__ >= 19
+#if __has_embed("../../VERSION.txt")
   static constinit auto version = [] () constexpr {
     // Read the version from version.txt at the root of the package's repo.
     char version[] = {
@@ -43,6 +44,10 @@ const char *swt_getTestingLibraryVersion(void) {
 #warning SWT_TESTING_LIBRARY_VERSION not defined and VERSION.txt not found: testing library version is unavailable
   return nullptr;
 #endif
+#else
+#warning SWT_TESTING_LIBRARY_VERSION not defined and could not read from VERSION.txt at compile time: testing library version is unavailable
+  return nullptr;
+#endif
 }
 
 void swt_getTestingLibraryCommit(const char *_Nullable *_Nonnull outHash, bool *outModified) {