build: depends macOS: hide linker visibility warnings #17839

Removes 3167 linker warnings when compiling for macOS.

Warnings like this one:

ld: warning: direct access in function 'boost::filesystem::detail::create_directories(boost::filesystem::path const&, boost::system::error_code*)' from file '/Users/rex/bitcoin/depends/x86_64-apple-darwin19.0.0/lib/libboost_filesystem-mt-x64.a(operations.o)' to global weak symbol 'boost::system::generic_category()::generic_category_instance' from file 'libbitcoin_util.a(libbitcoin_util_a-time.o)' means the weak symbol cannot be overridden at runtime. This was likely caused by different translation units being compiled with different visibility settings.

@rex4539 Please fill out the PR body and/or add more information to the commit itself. What warnings is this fixing, why is adding the -fvisibility options the correct fix etc.

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Gitian builds

I'd love to get rid of these warnings, but agree with @fanquake that this needs more explanation. Simply suppressing warnings can be risky.

https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/CppRuntimeEnv/Articles/SymbolVisibility.html

Withdrawing due to no interest.