ninja: Correctly handle copies steps in sourceless shared_library targets.

test/copies/src/foo.c

Index: test/copies/src/foo.c
diff --git a/test/copies/src/foo.c b/test/copies/src/foo.c
new file mode 100644
index 0000000000000000000000000000000000000000..99a4c103ba236c9204b21c3f503158c2a4f38989
--- /dev/null
+++ b/test/copies/src/foo.c
@@ -0,0 +1,13 @@
+// Copyright (c) 2015 Google Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+int f() { return 42; }
+
+#ifdef _MSC_VER
+// link.exe gets confused by sourceless shared libraries and needs this
+// to become unconfused.
+int __stdcall _DllMainCRTStartup(
+    unsigned hInst, unsigned reason, void* reserved) {
+  return 1;
+}
+#endif