Update macos installation instructions

[decentclock]

This should be merged after #536, and is a followup to #629

It includes build instructions for libsecp256k1 for those who don't use install.sh

@AdamISZ I was wrong, it turns out that install.sh does need some modifications to work on mac os from scratch.

My biggest question at this point is whether I have the list of darwin dependencies correct in install.sh: line 44

With the current list, I was able to build, and pass the testsuite on fresh installations of Mac OS Catalina with Apple Command Line Tools and Homebrew installed.

[AdamISZ]

@jameshilliard

Re @Jules23 's question above:

My biggest question at this point is whether I have the list of darwin dependencies correct in install.sh: line 44

With the current list, I was able to build, and pass the testsuite on fresh installations of Mac OS Catalina with Apple Command Line Tools and Homebrew installed.

Does this look right to you?

In absence of further comments I'm prepared to just merge it on a best effort basis if we don't have other testers.

Commits could be squashed perhaps, but it doesn't really matter.

[decentclock]

Rebase: squash commits, and fix omissions in INSTALL.md

[decentclock]

Implement feedback, test suite passes using latest install.sh

[AdamISZ]

Apart from that extra comment I made about the commit chosen, hopefully we can get this merged quickly (and squash the commits first).

The reason I'm especially keen to get this one done is, since we had the "hardfork" in the master branch, people need to re-run the installation so it'd be good to have the instructions/script correct.

[decentclock]

@AdamISZ I'm choosing the correct commit with the line
git reset --hard 0d9540b13ffcd7cd44cc361b8744b93d88aa76ba

I remember you had a cleaner way to do it, but couldn't find the syntax elsewhere.

[AdamISZ]

Yeah that works, or you can just checkout, but that does require git.

The way I did it (which I copied from other code somewhere) is just to set the .tar.gz filename. See how it's passed as an argument in libsecp256k1_install() in install.sh.

[AdamISZ]

Oh but since this is a manual install, I guess that's OK. Perhaps debatable, not sure.

[decentclock]

Do you want me to change it to how it's done in install.sh?

[AdamISZ]

It's fine. We've had manual instructions including a step like that before.

@jameshilliard @mecampbellsoup @kristapsk any further updates, comments? If not I'll merge this fairly shortly.

[kristapsk]

I could get my hands on a MacBook Pro at the weekend at test this myself probably.

[AdamISZ]

@kristapsk thank you. if you are going to do that, we may as well wait. But it won't be a disaster if you can't.

[kristapsk]

Tested install.sh on macOS Catalina 10.15.5, which didn't even had Homebrew installed before, it worked.