diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index e9b6dafbcb..abbc3d052b 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -1999,8 +1999,13 @@ void cmGeneratorTarget::ComputeModuleDefinitionInfo(
   info.WindowsExportAllSymbols =
     this->Makefile->IsOn("CMAKE_SUPPORT_WINDOWS_EXPORT_ALL_SYMBOLS") &&
     this->GetPropertyAsBool("WINDOWS_EXPORT_ALL_SYMBOLS");
+#if defined(_WIN32) && defined(CMAKE_BUILD_WITH_CMAKE)
   info.DefFileGenerated =
     info.WindowsExportAllSymbols || info.Sources.size() > 1;
+#else
+  // Our __create_def helper is only available on Windows.
+  info.DefFileGenerated = false;
+#endif
   if (info.DefFileGenerated) {
     info.DefFile = this->ObjectDirectory /* has slash */ + "exports.def";
   } else if (!info.Sources.empty()) {