Use gcc-9 for macOS builds

[smk762]

Closes #616

[dimxy]

LGTM

[DeckerSU]

LGTM also, but I need to conduct some tests on Mac, such as a full-sync, and other internal tests before I can give explicit approval. Therefore, it will be labeled as "Under Testing" until I complete all the necessary checks. I strongly believe that we should stop using gcc on Mac and switch to the native clang, as I have already done in the KomodoOcean repository. This is on my to-do list and I plan to open a separate PR for this till the next season. For now, we will continue with gcc@9.

[DeckerSU]

The compiler changes for Mac mentioned earlier are now available here: #618 .

[TheComputerGenie]

If the object is to update, why not update to a version that's modern enough to be supported in the future rather than the oldest working but most likely to be disabled version?
8 is no longer supported so move to 9 rather than 10, 11, 12, or 13?
I asked this in another post, but: Is there some logical reason that we observers are missing that justifies not making things for the future rather than so old that no one supports them? [or in this case will soon no longer support them]

[smk762]

If the object is to update, why not update to a version that's modern enough to be supported in the future rather than the oldest working but most likely to be disabled version? 8 is no longer supported so move to 9 rather than 10, 11, 12, or 13? I asked this in another post, but: Is there some logical reason that we observers are missing that justifies not making things for the future rather than so old that no one supports them? [or in this case will soon no longer support them]

In some cases, its for backwards compatibility so people using older operating systems which are still supported (i.e. not yet at EOL) can continue use the app/binary etc. Another consideration is resulting required code / syntax changes which the dependency being updated has breaking changes, or triggers a cascade of other deps needing updates or deps being incompatible if one doesnt support the other yet. Resource allocation is also a consideration. If I can change a number in a few files and it works, I dont need to spend hours/days on a while bunch of other changes trying to make it work.