a52ecc936a build: set minimum supported macOS to 10.14 (fanquake)
Pull request description:
This is a requirement for C++17 support. See my comments [here](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-643722538):
> You cannot use std::get with std::variant on macOS < 10.14, because Apples libc++ doesn't support the std::bad_variant_access exception. [Relevant comment](https://github.com/bitcoin/bitcoin/pull/19183#discussion_r439794318) in #19183.
> While we could work around this in our own code, using std::get_if, this would still be a problem for 3rd-party dependencies.
> I've been testing Qt 5.15LTS (we'll have to enable C++17 in qt, and may upgrade to a newer version at the same time), and you can't enable -std c++17, while targeting a macOS deployment version < 10.14, configuring will fail. They are making use of std::get with std::variant throughout their cocoa code.
We would have to had to have bumped to at least 10.13 in any case, as Qt 5.15 (#19716) [requires 10.13+](https://doc.qt.io/qt-5/supported-platforms.html).
ACKs for top commit:
hebasto:
ACK a52ecc936a, I have reviewed the code and it looks OK, I agree it can be merged.
Tree-SHA512: f669b2fc777aeea1e9afdbbc7bd9afe3997418211db6ba53c934cae0e62a9b999603da539518c229f34961d275c9e2f315c7b022cf5fb97bd201a69c85d470cc
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