Previously, libgo relied on the _sigev_un implementation-specific field in struct sigevent, which is only available on glibc. This patch uses the sigev_notify_thread_id macro instead which is mandated by timer_create(2). In theory, this should work with any libc implementation for Linux. Unfortunately, there is an open glibc bug as glibc does not define this macro. For this reason, a glibc-specific workaround is required. Other libcs (such as musl) define the macro and don't require the workaround. See https://sourceware.org/bugzilla/show_bug.cgi?id=27417 This makes libgo compatible with musl libc. Based on patch by Sören Tempel. Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/434755