Greatly reduce build warning spam.

No-one ever fixes unused-* or deprecated-* warnings in external/ (nor should they!).

-Wno-deprecated-enum-enum-conversion should probably have been added
next to -Wno-deprecated-anon-enum-enum-conversion when it was added,
since it's a major spam contributor, and equally unlikely that anyone
will clean them up.

-Wno-deprecated-dynamic-exception-spec is a weird special case because
it should be irrelevant after our next libc++ update, but in the
meantime -- because libc++ is used everywhere -- this is a massive spam
contributor that no-one on Android cares about because we don't use
exceptions (and the people seeing this can't fix libc++ anyway).

Test: local clean builds
Change-Id: I098202337e9e5026c4c5215dbf5a2abf1fbbdca8