GN: fixes for Mac

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 1ddf12a9579855a38fdfe746e655ca0bb2958514..07c70097afcec761e09065957226ce99462a76d4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -19,7 +19,7 @@ skia_public_includes = [
   "include/utils",
   "include/utils/mac",
 
-  "include/c",   # TODO: move back to top, order shouldn't matter
+  "include/c",  # TODO: move back to top, order shouldn't matter
 ]
 
 # Skia public API, generally provided by :skia.
@@ -148,6 +148,8 @@ component("skia") {
     ":opts_avx",
     ":opts_sse41",
     ":opts_ssse3",
+    "//third_party/libjpeg_turbo",
+    "//third_party/libpng",

[abarth, 2016/07/27 16:59:00]

We don't have these libs yet, but when we do have them, hits will work properly.
 Please don't feel blocked on us.  We've got Skia pinning to a rev and we'll
backfill these sorts of things when we bump up the revision we use.

[mtklein, 2016/07/27 17:04:07]

Thanks, sounds good.

(The versions you see in our GN files are pretty bare-bones... none of the
platform specific code.  Should be safe to steal, but not optimal.)

     "//third_party/zlib",
   ]
 
@@ -221,8 +223,6 @@ component("skia") {
     deps += [
       "third_party:fontconfig",
       "third_party:freetype2",
-      "third_party:libjpeg-turbo",
-      "third_party:libpng",
     ]
     sources += [
       "src/fonts/SkFontMgr_fontconfig.cpp",
@@ -262,7 +262,10 @@ action("skia.h") {
 
 executable("fiddle") {
   include_dirs = [ "$target_gen_dir" ]
-  libs = [ "OSMesa" ]
+  libs = []
+  if (is_linux) {
+    libs += [ "OSMesa" ]
+  }
 
   sources = [
     "tools/fiddle/draw.cpp",