macOS: Add additional flags to OSLIBS

[benz0li]

• Remove file src/mach_dyld_atfork.tbd
• Fixes Alternative to copying part of a recent Xcode libSystem.tbd to src/mach_dyld_atfork.tbd #49976

[benz0li]

Cross reference: macports/macports-ports#18905

Same patch. But only applied if darwin version < 19 (macOS version < 10.15 (Catalina)).

[benz0li]

@quentinmit The MacPorts build for macOS 10.14 (Mojave) now succeeds.
ℹ️ It does not build for older versions ('OS X'/macOS 10.10 - 10.13 (Yosemite - High Sierra)) on MacPorts, though.

[benz0li]

@vtjnash I am currently running the MacPorts build for macOS 10.13 (High Sierra) locally.

If it succeeds, this can be merged.

[benz0li]

@vtjnash One more thing: Is Julia for macOS 10.10+ x86-64 (64-bit) built with macOS10.15 using Xcode 12.4 12D4e and Apple clang version 12.0.0 (clang-1200.0.32.29)?

Because MacPorts builds for 'OS X'/macOS 10.10 - 10.13 (Yosemite - High Sierra) fail due to Apple clang versions < 9.3 (and MacPorts clang somehow not fully meeting the requirements).
❓ Is Julia for Linux (Glibc) 2.6.24+ x86-64 (64-bit) built using gcc?

[vtjnash]

It looks like it has the wrong version of LLVM, from the failure in that build log. Not sure how that could have happened though. Yes, linux is normally built with gcc (though can be built with clang).