diff --git a/src/include/c.h b/src/include/c.h
index 4f6ce0c99ae..774a0a15497 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -53,6 +53,15 @@
 #include "pg_config.h"
 #include "pg_config_manual.h"	/* must be after pg_config.h */
 
+/*
+ * We always rely on the WIN32 macro being set by our build system,
+ * but _WIN32 is the compiler pre-defined macro. So make sure we define
+ * WIN32 whenever _WIN32 is set, to facilitate standalone building.
+ */
+#if defined(_WIN32)
+#define WIN32
+#endif
+
 #if !defined(WIN32) && !defined(__CYGWIN__)		/* win32 includes further down */
 #include "pg_config_os.h"		/* must be before any system header files */
 #endif