diff --git a/Source/cmNMakeMakefileGenerator.cxx b/Source/cmNMakeMakefileGenerator.cxx
index 713cc697c..ba808f011 100644
--- a/Source/cmNMakeMakefileGenerator.cxx
+++ b/Source/cmNMakeMakefileGenerator.cxx
@@ -137,9 +137,9 @@ void cmNMakeMakefileGenerator::OutputMakeVariables(std::ostream& fout)
     "# Path to cmake\n"
     "MAKESILENT                             = /nologo\n"
     "CMAKE_STANDARD_WINDOWS_LIBRARIES       = @CMAKE_STANDARD_WINDOWS_LIBRARIES@\n"
-    "CMAKE_C_FLAGS                          = @CMAKE_C_FLAGS@ @BUILD_FLAGS@\n"
+    "CMAKE_C_FLAGS                          = @CMAKE_C_FLAGS@ @BUILD_FLAGS@ -DWIN32\n"
     "CMAKE_C_LINK_EXECUTABLE_FLAG           = @CMAKE_C_LINK_EXECUTABLE_FLAG@\n"
-    "CMAKE_CXX_FLAGS                        = @CMAKE_CXX_FLAGS@ @BUILD_FLAGS@\n"
+    "CMAKE_CXX_FLAGS                        = @CMAKE_CXX_FLAGS@ -DWIN32 @BUILD_FLAGS@\n"
     "CMAKE_LINKER_FLAGS                     = @CMAKE_LINKER_FLAGS@ @LINKER_BUILD_FLAGS@\n"
     "CMAKE_LINKER_SHARED_LIBRARY_FLAG       = @CMAKE_LINKER_SHARED_LIBRARY_FLAG@\n"
     "CMAKE_LIBRARY_MANAGER_FLAGS            = @CMAKE_LIBRARY_MANAGER_FLAGS@\n"