GN: fixes for Mac

third_party/libpng/BUILD.gn

Index: third_party/libpng/BUILD.gn
diff --git a/third_party/libpng/BUILD.gn b/third_party/libpng/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..fd80aad3b9718b580da371dd2a658f2efdc2e1e6
--- /dev/null
+++ b/third_party/libpng/BUILD.gn
@@ -0,0 +1,37 @@
+# Copyright 2016 Google Inc.
+#
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+declare_args() {
+}
+
+import("../third_party.gni")
+
+config("libpng_public") {
+  include_dirs = [ "." ]
+}
+third_party("libpng") {
+  public_configs = [ ":libpng_public" ]
+
+  deps = [
+    "//third_party/zlib",
+  ]
+  sources = [
+    "png.c",
+    "pngerror.c",
+    "pngget.c",
+    "pngmem.c",
+    "pngpread.c",
+    "pngread.c",
+    "pngrio.c",
+    "pngrtran.c",
+    "pngrutil.c",
+    "pngset.c",
+    "pngtrans.c",
+    "pngwio.c",
+    "pngwrite.c",
+    "pngwtran.c",
+    "pngwutil.c",
+  ]
+}