Dependencies

I think the dependencies might be overly strict. For example,

* On Mac, it requires full xcode, but I only have xcode command line tools and always compile gtsam (manually from source) just fine

* On a linux machine I tested on, it said

> Warning: No available formula with the name "g++" (dependency of borglab/core/gtsam_develop).
==> Searching for similarly named formulae...

But the machine has g++ installed so idk I guess brew doesn't know about it or something.

Inspect the dependency definition for borglab/core/gtsam_develop and reproduce the reported checks on macOS with Xcode command line tools and on Linux with g++ installed. Confirm whether the formula incorrectly requires full Xcode or treats g++ as an unavailable Homebrew formula; done means the dependency checks accept both supported setups.

Written by the indexing model from the issue text.