Extract INIT_PTHREAD_ATTR_GET_SCHED from INIT_PTHREAD_ATTR_GET,
as the former is not supported on OpenBSD.
Supported interceptors
getdetachstate, getguardsize, getscope, getstacksize, getgrouplist and getstack
Unsupported
getschedparam, getgroupmembership and getschedpolicy
Patch by David CARLIER
CMakeLists.txt change should be in the patch which adds this files