Protobuf & Abseil compilation pollutes build with a lot of warnings

[vlstill]

Compiling with FetchContent Abseil and Protobuf, there is a lot of warnings. In parallel build they are also interleaved with proper p4c/extensions build which makes readability even worse.

• We should supress warnings in the builds of dependencies, they are not relevant.
• We should make sure the abseil/protobuf include search paths are included as -isystem -- some of the warnings are coming from abseil headers included into our code.

[fruffy]

I have been trying to make Abseil a system include here: #4511

The problem is the overly opinionated Bazel build. We may have to patch the Abseil Bazel dependency to get things to work because I do not think the Abseil devs will fix this.

Relevant issue:
abseil/abseil-cpp#740

Edit: Someone suggested a patch which may work: abseil/abseil-cpp#1637