use -isystem for headers from third_party

This supresses warnings from code in those headers,
e.g. warnings about switch case fallthrough in SDL_memset4(),
defined inline in SDL_stdinc.h.

Change-Id: I5341a67d4949b28ec5ffa6b7ae433748406e99db
Reviewed-on: https://skia-review.googlesource.com/109140
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed