[CMake] Localize a build hack

[davezarzycki]

The '-Wl,-z,defs' flag is useful, so let's disable it only where needed.

[davezarzycki]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 11060d4

[davezarzycki]

The only failure in the full test was/is the known and unrelated ModelIO.swift failure. Let's smoke test this PR to bypass that problem and be merge ready.

@swift-ci please smoke test

[davezarzycki]

@compnerd -- Does this patch look okay with you?

[compnerd]

Why do you suspect that this only is an issue for the SwiftRemoteMirror? I suspect that this breaks the tests too for Windows when cross compiling. I’d say if you want -z defs, you should do this at a much deeper layer in swift_add_library with ELF style output.

[davezarzycki]

Hi @compnerd – A few thoughts and questions:

1. Do we agree that "-Wl,-z,defs" is a good thing? As a reminder, it forces the linker to not ignore missing definitions/symbols.
2. In my Linux ELF system testing, SwiftRemoteMirror is the only library that "needs" the flag removed, and I think even that case is fixable.
3. I don't have access to a Windows machine, nor are Windows machines a part of Swift's continuous integration, so any advice that isn't speculative would be helpful. Would you be open to trying this patch out? We can always revert it if Windows needs the flag removed in more scenarios.
4. At the very least, I'd like to make the top level removal of this flag conditional on platforms that need it. What do you think of this option?

[davezarzycki]

On second thought, I'll just preserve the Windows behavior. Please consider reviewing #12895 instead. Thanks!