Changes to the OS X build documentation

[Adam Weber <ad...@gmail.com>]

Hey all, I tried to follow the OS X build guidelines and ran into
these couple of issues.  I couldn't seem to find where this document
is generated so I figured I'd just email here.

I found these two issues on my 10.13.6 machine.  Perhaps they're not
real issues and I understand OS X isn't high priority, but figured I'd
send this here because someone else might run into it.

$ brew install gnu-sed --with-default-names #The macOS default sed
handles the -i parameter differently

solution: follow the path instructions output at the end of installation.

$ thirdparty/build-if-necessary.sh

dyld: Library not loaded: /usr/local/opt/mpfr/lib/libmpfr.4.dylib

  Referenced from: /usr/local/bin/awk

  Reason: image not found

solution: brew upgrade gawk

Let me know if those are unclear!
--
Adam