Fix compile issues in sky on android

sky/engine/BUILD.gn

Index: sky/engine/BUILD.gn
diff --git a/sky/engine/BUILD.gn b/sky/engine/BUILD.gn
index 71e9255ef008e9abf037e674021fc407cad1f6d7..120061999da1ed2782a88c1f927b3155ac3ce3be 100644
--- a/sky/engine/BUILD.gn
+++ b/sky/engine/BUILD.gn
@@ -6,6 +6,9 @@ import("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
+if (is_posix) {
+  import("//build/config/gcc/gcc_version.gni")
+}
 
 visibility = ["//sky/*"]
 
@@ -64,6 +67,10 @@ config("config") {
     cflags += [ "-Wno-uninitialized" ]
   }
 
+  if (is_posix && gcc_version >= 49) {
+    cflags += [ "-Wno-unused-but-set-variable" ]
+  }
+
   if (is_clang) {
     if (sky_gc_plugin && clang_use_chrome_plugins) {
       # TODO(GYP)