Code review comment for lp:~laurynas-biveinis/percona-server/xtradb-thread-priority-flag

Just make it Linux-specific for now, i.e. so that the system variables only appear in UNIV_LINUX builds, and srv_current_thread_priority is #define'd to 0 otherwise.

Making it truly portable is more complex than just #ifdef __GNUC__, and I will address it later.

But I also don't like reviewing this change in isolation. Please resubmit with the code that actually makes use of srv_current_thread_priority.