Tofino dependencies

[Nieuwejaar]

This adds two packages that are needed to build the tofino SDE. The CI scripts for that build are currently pulling pre-built packages from an S3 bucket, but it would be cleaner and more sustainable if they could be pulled from the helios package repo instead.

[citrus-it]

I've pushed some changes that tweak things a bit and get this closer to something we could upstream to OmniOS, which would make future helios updates easier.

This consists of:

• use the supplied functions like set_arch and set_builddir as they have other side effects;
• build both 32-bit and 64-bit abseil versions and install into the standard location (which was previously prevented by protobuf);
• fix protobuf to use the abseil package and stop delivering its own copy;
• fix a couple of build errors in boost and revert the small change to lib/functions.sh so we don't diverge there.

@Nieuwejaar please take a look. I suspect the build failures just weren't noticed previously as the exit status of b2 was not being checked. They've already been fixed upstream and we can carry a patch for now.

[jclulow]

I'm curious about the ABI stability of these things, as the versions go up. Do you have a sense of whether abseil or boost remain backwards compatible as they release new versions?

[bnaecker]

On that front, the oxide/build.sh script we're using hard-codes a CMake variable to using a local copy of Abseil. But one could download it from a specific source revision for the build itself, which we might want to do for ABI compatibility reasons.