Allow Bazel users to include Abseil headers with angle brackets.

[coryan]

Because we use Abseil with both CMake and Bazel, and we sometimes use Abseil from /usr/include (or /usr/local/include or some other $PREFIX) we need to always include Abseil headers with either quotes (#include "absl/...") or angle brackets (#include <absl/...>).

We would prefer to include Abseil with angle brackets. In some platforms that allows us to treat warnings as errors (-Werror -Wall -W... or /WX /W3) even if the Abseil code is not "clean" for some warning. In general Abseil is very very clean, thanks! But there are a few exceptions, e.g. #582 .

[derekmauro]

@EricWF are you still interested in working on this?

[fruffy]

We are also running into this problem. Other Google repositories such as gRPC and Protobuf supported bracketed includes, but abseil does not.