Fix build on macOS < 10.11

[barracuda156]

Implementation used for Apple here can only work for 10.11+: https://developer.apple.com/documentation/kernel/sa_endpoints_t
Drop to fallback for older versions.

[salzmdan]

Thank you for the patches, but I don't understand why to support such old versions (https://endoflife.date/macos). We don't support newer versions either, such as Ubuntu 18.04.

[barracuda156]

@salzmdan Thank you for responding. Re supporting:

1. There are active users and some developers even on 10.4. Not many, but they exist. (There are some specific reasons to use 10.4–10.6 too.)
2. GCC upstream supports 10.4+ (testing on 10.5+), CMake and Boost support 10.4+ (no pro-active testing, but accept tickets and patches), MacPorts support 10.5+ (10.4 was dropped in the current release, so there are a few years of support ahead for 10.5–10.6), pkgsrc has some (limited) support for 10.4+. So the platform has everything needed, including modern toolchain and a lot of software.
3. As long as patches are minimal, why not improve the code, even for a non-mainstream platform.