This fixes the build on Linux (dep on third_party:zlib -> third_party/zlib).
I've moved declare_args() {} back to each .gn file... seems like args want
to be as local as possible in GN land.
Additionally, refactor all the common third_party config and warning flag
changes into a template, third_party. This lets it all live together in a
.gni: at head unwanted_configs can be in a .gni (it's just a variable) but
config("no_warnings") (and thus third_party_configs) cannot, as configs
cannot be part of .gni files.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2163653002
Review-Url: https://codereview.chromium.org/2163653002
