Make "win" a separate port from "cygwin". This means you can now

configure under native Windows (MinGW that is), but you won't get very far
compiling yet.  The dynaloader files are from Jan Wieck's patch set.

src/backend/port/dynloader/cygwin.h

@@ -0,0 +1,36 @@
+/*-------------------------------------------------------------------------
+ *
+ * Dynamic loader declarations for Cygwin
+ *
+ * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $Header: /cvsroot/pgsql/src/backend/port/dynloader/cygwin.h,v 1.1 2003/03/21 17:18:34 petere Exp $
+ *
+ *-------------------------------------------------------------------------
+  */
+#ifndef PORT_PROTOS_H
+#define PORT_PROTOS_H
+
+#include <dlfcn.h>
+#include "utils/dynamic_loader.h"
+
+/*
+ * In some older systems, the RTLD_NOW flag isn't defined and the mode
+ * argument to dlopen must always be 1.  The RTLD_GLOBAL flag is wanted
+ * if available, but it doesn't exist everywhere.
+ * If it doesn't exist, set it to 0 so it has no effect.
+ */
+#ifndef RTLD_NOW
+#define RTLD_NOW 1
+#endif
+#ifndef RTLD_GLOBAL
+#define RTLD_GLOBAL 0
+#endif
+
+#define pg_dlopen(f)	dlopen((f), RTLD_NOW | RTLD_GLOBAL)
+#define pg_dlsym		dlsym
+#define pg_dlclose		dlclose
+#define pg_dlerror		dlerror
+
+#endif   /* PORT_PROTOS_H */