Add some platform abstractions.

(This is a merge of http://go/wvgerrit/70303)

This adds a platform.h file to abstract some of the differences
between Windows and POSIX platforms.  This includes ntohl, setenv,
and ssize_t.

Bug: 122953649
Test: Android Unit Tests
Change-Id: I3235f3f284b53d24d7365ff3f4a06dcd9b403697

libwvdrmengine/cdm/util/src/platform.cpp

@@ -0,0 +1,21 @@
+// Copyright 2018 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine Master
+// License Agreement.
+
+#include "platform.h"
+
+#include "stdlib.h"
+
+#ifdef _WIN32
+
+int setenv(const char* key, const char* value, int overwrite) {
+  if (!overwrite) {
+    size_t size;
+    errno_t err = getenv_s(&size, nullptr, 0, key);
+    if (err != 0 || size != 0)
+      return err;  // Return 0 if it exists, but don't change.
+  }
+  return _putenv_s(key, value);
+}
+
+#endif