Summary of changes:
- Vendor OTP at: rel/vendor/otp
- Remove rel/vendor/otp/lib/*/src
- Include /usr/local/bin in $PATH. This is not there by default and that's where homebrew (on x86_64) installs things so it should make it easier to build some packages with NIFs.
Since we're building our own OTP for Mac, the idea was not to
accidentally building agains something we happen to be testing at the
moment, like the latest RC. This check turned out to be a little bit
too annoying in practice though. We plan to automate builds on CI
in the near future so we wouldn't need it the anyway.
Closes#1177