cmake HAVE_THREAD_RWLOCK_ARCH_T_NR_READERS examples

darktable-org/darktable ConfigureChecks.cmake :68 

check_struct_has_member(
  "struct __pthread_rwlock_arch_t" "__nr_readers" "pthread.h"
  HAVE_THREAD_RWLOCK_ARCH_T_NR_READERS LANGUAGE C)