a43b8e9555 build: set OSX_MIN_VERSION to 10.15 (fanquake)
Pull request description:
Taken out of #20744, as splitting up some of the build changes was mentioned [here](https://github.com/bitcoin/bitcoin/pull/22937#discussion_r707303172).
This is required to use `std::filesystem` on macOS, as support for it only landed in the libc++.dylib shipped with 10.15. So if we want to move to using `std::filesystem` for `23.0`, this bump is required.
See also: https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes
> Clang now supports the C++17 \<filesystem\> library for iOS 13, macOS 10.15, watchOS 6, and tvOS 13.
macOS 10.15 was released in October 2019. macOS OS's seem to have a life of about 3 years, so it's possible that 10.14 will become officially unsupported by the end of 2021 and prior to the release of 23.0.
Guix builds:
```bash
bash-5.1# find guix-build-$(git rev-parse --short=12 HEAD)/output/ -type f -print0 | env LC_ALL=C sort -z | xargs -r0 sha256sum
abc8b749be65f1339dcdf44bd1ed6ade2533b8e3b5030ad1dde0ae0cede78136 guix-build-a43b8e955558/output/dist-archive/bitcoin-a43b8e955558.tar.gz
1edcc301eb4c02f3baa379beb8d4c78e661abc24a293813bc9d900cf7255b790 guix-build-a43b8e955558/output/x86_64-apple-darwin19/SHA256SUMS.part
e9dbb5594a664519da778dde9ed861c3f0f631525672e17a67eeda599f16ff44 guix-build-a43b8e955558/output/x86_64-apple-darwin19/bitcoin-a43b8e955558-osx-unsigned.dmg
11b23a17c630dddc7594c25625eea3de42db50f355733b9ce9ade2d8eba3a8f3 guix-build-a43b8e955558/output/x86_64-apple-darwin19/bitcoin-a43b8e955558-osx-unsigned.tar.gz
257ba64a327927f94d9aa0a68da3a2695cf880b3ed1a0113c5a966dcc426eb5e guix-build-a43b8e955558/output/x86_64-apple-darwin19/bitcoin-a43b8e955558-osx64.tar.gz
```
ACKs for top commit:
hebasto:
ACK a43b8e9555
jarolrod:
ACK a43b8e9
Tree-SHA512: 9ac77be7cb56c068578860a3b2b8b7487c9e18b71b14aedd77a9c663f5d4bb19756d551770c02ddd12f1797beea5757b261588e7b67fb53509bb998ee8022369
Passing ADDITIONAL_GUIX_COMMON_FLAGS="--no-substitutes --bootstrap" as
suggested doesn't work:
```bash
...outputting in: '/bitcoin/guix-build-a1f0b8b62eb8/output/x86_64-linux-gnu'
...bind-mounted in container to: '/outdir-base/x86_64-linux-gnu'
guix time-machine: error: bootstrap: unrecognized option
```
and I think bootstrapping is more than covered in the preceding "Choose
your security model" section.
In Guix, there are two flags for controlling parallelism:
Note: When I say "derivation," think "package"
--cores=n
- controls the number of CPU cores to build each derivation. This is
the value passed to `make`'s `--jobs=` flag.
- defaults to 0: as many cores as is available
--max-jobs=n
- controls how many derivations can be built in parallel
- defaults to 1
Therefore, if set --max-jobs=$MAX_JOBS and don't set --cores, Guix could
theoretically spin up $MAX_JOBS * $(nproc) number of threads, and that's
no good.
So we could either default to --cores=1, --max-jobs=$MAX_JOBS
- Pro: --cores=1 means that `make` will be invoked with `-j1`,
avoiding problems with package whose build systems and test
suites break when running multi-threaded.
- Con: There will be times when only 1 or 2 derivations can be built
at a time, because the rest of the dependency graph all depend
on those 1 or 2 derivations. During these times, the machine
will be severely under-utilized.
or --cores=$MAX_JOBS, --max-jobs=1
- Pro: We don't encounter prolonged periods of
severe under-utilization mentioned above.
- Con: Many packages' build systems and test suites break when running
multi-threaded.
or --cores=1, --max-jobs=1 and let the user override with
$ADDITIONAL_GUIX_COMMON_FLAGS
- Clearer and more accurate prose
- Pin `guix pull' to commit rather than branch
- Just use `use-module' instead of `define-module'
- Use `bash-minimal' instead of `bash'
- Remove unneeded `tcsh' from manifest
- Explicitly use `python-3.7'
- Add comments about how {native,cross}-toolchains are produced and
why
