Allow most third_party targets to use system libraries if asked.

This extends the pattern in freetype2 to expat, icu, libjpeg-turbo, libpng, libwebp, and zlib, and gives all these an arg to control which to use.

Homebrew doesn't have dng_sdk, piex, or sftnly, or I'd have done the same for them too.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4260

DOCS_PREVIEW= https://skia.org/?cl=4260

Change-Id: I82e780502bf2217336e791787f172a6fc8f55460
Reviewed-on: https://skia-review.googlesource.com/4260
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Hal Canary <halcanary@google.com>
10 files changed