Enable both static and dynamically linked libpng

src/codec/SkCodec_libpng.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
 #include "SkEncodedFormat.h"
 #include "SkImageInfo.h"
 #include "SkRefCnt.h"
 #include "SkSwizzler.h"
 
-extern "C" {
-    #include "png.h"
-}
+#ifdef SKIA_PNG_PREFIXED
+    // this must proceed png.h
+    #include "pngprefix.h"
+#endif
+#include "png.h"
 
 class SkScanlineDecoder;
 class SkStream;
 
 class SkPngCodec : public SkCodec {
 public:
     // Assumes IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*);
     static bool IsPng(SkStream*);
 protected:
@@ skipping 23 matching lines @@
     // Calls rewindIfNeeded, and returns true if the decoder can continue.
     bool handleRewind();
     bool decodePalette(bool premultiply);
     void finish();
     void destroyReadStruct();
 
     friend class SkPngScanlineDecoder;
 
     typedef SkCodec INHERITED;
 };