Rather than trying to encode all of the rules in a header, lets just put
them in the build system where they belong. This fixes the build on
FreeBSD, which does have pthraed_setaffinity_np, but it's in a
pthread_np.h, not behind _GNU_SOURCE. FreeBSD also implements cpu_set
slightly differently, so additional changes would be required to get it
working right there anyway.

v2: - fix #define in autotools

Fixes: 9f1bbbdb


       ("util: try to fix the Android and MacOS build")
Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>