OSX build failures

[ekg]

@jeizenga mentioned that it's not possible to build xg on OSX anymore.

I can confirm that the problem isn't clang. I had no problem building this way:

export CC=$(which clang); export CXX=$(which clang++); cmake -H. -Bbuild && cmake --build build -- -j 3

So what's the issue?

[jeizenga]

It looks to be a linking error to me, but there's a lot to parse. Here's the error output:

https://docs.google.com/document/d/1QugcBVgFGNM_peFexsa99vuDjczAFdzwHCf6FwDnNcg/edit?usp=sharing