add Windows platform wrapper

src/WindowsPlatform.hh

@@ -0,0 +1,15 @@
+#pragma once
+
+// <windows.h> defines a lot of annoying things like ERROR and DELETE, which
+// we use as names in enum classes, so we wrap <windows.h> with this header
+// that undoes some of these annoying things.
+
+#include <phosg/Platform.hh>
+
+#ifdef PHOSG_WINDOWS
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#undef DELETE
+#undef ERROR
+#undef PASSTHROUGH
+#endif