Allow for non-vendored abseil and googletest

[timkpaine]

Many registries don't like it when you vendor third party, so we should make these opt-out.

ref: Homebrew/homebrew-core#142146 (comment)

[pieter3d]

I don't follow, what does it mean to "vendor" third_party?

[timkpaine]

I can do this so no worries, but basically don't require git submodules, instead either be entirely self contained or otherwise able to pull from the user's existing installed libraries.

[pieter3d]

I see. Submodules seem like a nice solution though. If you rely on a user's local install, you can have all kinds of random compilation issues. Surelog itself is full of submodules too.

[timkpaine]

Most registries don't want this type of thing. Surelog doesn't require any submodules either, it will happily pull them from your outer environment