write -isystem paths relative to the build root

The default makes these system-absolute paths,
which confuses fiddle's overlay builds.

Now we should see things like,

  -isystem ../third_party/externals/jsoncpp/include

where we previously had,

  -isystem /Users/mtklein/skia/third_party/externals/jsoncpp/include

Change-Id: I7b161a550fdb95b06e17c372cd5bec3015e3c8b7
Reviewed-on: https://skia-review.googlesource.com/109382
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

third_party/third_party.gni

@@ -13,7 +13,7 @@ template("third_party") {
     } else {
         cflags = []
         foreach(dir, invoker.public_include_dirs) {
-            cflags += ["-isystem", rebase_path(dir)]
+            cflags += ["-isystem", rebase_path(dir, root_build_dir)]
         }
     }
   }