If you attempt to build this package on macOS using LLVM/Clang 11.0 (the upstream version number, not Apple's version numbers, which are different), you get the following error:

cpu/cpu_darwin_cgo.go:13:5: error: 'TARGET_OS_MAC' is not defined, evaluates to 0 [-Werror,-Wundef-prefix=TARGET_OS_]
#if TARGET_OS_MAC
    ^
1 error generated.

This is because LLVM/Clang 11 automatically adds -Werror,-Wundef-prefix=TARGET_OS_ as default compiler arguments and apparently TARGET_OS_MAC isn't defined in this compilation environment.

The reason it is not defined, I'm not sure. TargetConditionals.h defines this symbol and unless I'm looking at the wrong SDK, I think TARGET_OS_MAC should always be defined when building on/for macOS.

I can work around this by adding #cgo CFLAGS: -Wno-undef-prefix or #include <TargetConditionals.h> to the failing import "C" comment. The latter is likely more robust and seemingly indicates that TargetConditions.h isn't being included by another #include in this block.

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.15.7
BuildVersion:   19H2

I'm also building with the macosx10.15 SDK.