Revert -isystem related CLs.

Revert "use -isystem for headers from third_party"
    This reverts commit 138bd155ed.

    Revert "write -isystem paths relative to the build root"
    This reverts commit 085bc52363.

I suspect the first breaks building on systems with an old libpng
or zlib installed, and the second definitely breaks GN -> CMake.

Change-Id: I5b29669b21b1444daeec8fb784337422ee17311a
Reviewed-on: https://skia-review.googlesource.com/110183
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

third_party/third_party.gni

@@ -8,14 +8,7 @@ template("third_party") {
     if (defined(invoker.public_defines)) {
       defines = invoker.public_defines
     }
-    if (is_win) {
-        include_dirs = invoker.public_include_dirs
-    } else {
-        cflags = []
-        foreach(dir, invoker.public_include_dirs) {
-            cflags += ["-isystem", rebase_path(dir, root_build_dir)]
-        }
-    }
+    include_dirs = invoker.public_include_dirs
   }
   source_set(target_name) {
     forward_variables_from(invoker, "*", [ "public_include_dirs" ])