CMake, include/:  changes to make fiddle 2.0 better

cmake: remove unused directories from skia.h / ${public_includes}

SkPreConfig.h:  work around buggy `#ifdef linux`

include:  guard many platform-specific headers
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac10.9-Clang-x86_64-Release-CMake-Trybot

Review URL: https://codereview.chromium.org/1488813002
diff --git a/include/ports/SkTypeface_mac.h b/include/ports/SkTypeface_mac.h
index 59bf77b..14440b5 100644
--- a/include/ports/SkTypeface_mac.h
+++ b/include/ports/SkTypeface_mac.h
@@ -10,6 +10,8 @@
 
 #include "SkTypeface.h"
 
+#if defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
+
 #include <CoreFoundation/CoreFoundation.h>
 
 #ifdef SK_BUILD_FOR_MAC
@@ -43,4 +45,5 @@
  */
 SK_API extern CTFontRef SkTypeface_GetCTFontRef(const SkTypeface* face);
 
-#endif
+#endif  // defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
+#endif  // SkTypeface_mac_DEFINED