Instead of always supplying flags, apply the flags as defaults. Use
clang's native flags instead of lifting the linker flags from binutils
with `-Wl,`.
If a project is using clang to drive linking, make clang do the right
thing with MACOSX_DEPLOYMENT_TARGET. This can be overridden by command
line arguments. This will cause modern clang to pass
`-platform_version 10.12 0.0.0`, since it doesn't know about the SDK
settings. Older versions of clang will pass down `-macos_version_min`
flags with no sdk version.
At the linker layer, apply a default value for anything left
ambiguous. If nothing is specified, pass a full
`-platform_version`. If only `-macos_version_min` is specified, then
lock down the sdk_version explicitly with `-sdk_version`. If a min
version and sdk version is passed, do nothing.
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