Turn on flags to enforce SK_API.

gn/BUILD.gn

 # 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() {
   ar = "ar"
   cc = "cc"
   cxx = "c++"
 }
 
 config("default") {
   cflags = [
     "-g",
     "-fstrict-aliasing",
     "-fPIC",
+    "-fvisibility=hidden",
 
     "-Werror",
     "-Wall",
     "-Wextra",
     "-Winit-self",
     "-Wpointer-arith",
     "-Wsign-compare",
     "-Wvla",
 
     "-Wno-deprecated-declarations",
     "-Wno-unused-parameter",
   ]
   cflags_cc = [
     "-std=c++11",
     "-fno-exceptions",
     "-fno-rtti",
     "-fno-threadsafe-statics",
+    "-fvisibility-inlines-hidden",
 
     "-Wnon-virtual-dtor",
   ]
 }
 
 config("release") {
   cflags = [ "-Os" ]
   defines = [ "NDEBUG" ]
 }
 
@@ skipping 69 matching lines @@
   }
 
   tool("stamp") {
     command = "touch {{output}}"
   }
 
   tool("copy") {
     command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
   }
 }