This change disables -Werror,-Wbuiltin-requires-header when compiling with clang.
Clang 8 is getting confused about having to include Clang's built-in "pthread.h" while it's trying to compile FreeBSD "pthread.h" itself. The error bellow repeats mentioning "pthread_create" on many other places. (similar issue https://lkml.org/lkml/2019/1/11/718)
PR: 235583