Enable priority inheritance for RTT mutexes (gnulinux)

[meyerj]

By default pthread mutexes used in the implementation of the gnulinux target do not have priority inheritance enabled. Priority inheritance is one way to prevent priorty inversion issues in real-time systems. So this is a pretty important feature for Orocos RTT, too.

The behavior of setting the attribute PTHREAD_PRIO_INHERIT is well explained in the POSIX documentation for pthread_mutexattr_setprotocol:

The protocol attribute defines the protocol to be followed in utilizing mutexes. The value of protocol may be one of:

PTHREAD_PRIO_INHERIT
PTHREAD_PRIO_NONE
PTHREAD_PRIO_PROTECT

which are defined in the <pthread.h> header. The default value of the attribute shall be PTHREAD_PRIO_NONE.

When a thread owns a mutex with the PTHREAD_PRIO_NONE protocol attribute, its priority and scheduling shall not be affected by its mutex ownership.

When a thread is blocking higher priority threads because of owning one or more non-robust mutexes with the PTHREAD_PRIO_INHERIT protocol attribute, it shall execute at the higher of its priority or the priority of the highest priority thread waiting on any of the non-robust mutexes owned by this thread and initialized with this protocol.

[...]

Xenomai mutex services enforce a priority inheritance protocol in order to solve priority inversions (https://xenomai.org/documentation/xenomai-2.6/html/api/group__mutex.html).

I am not sure whether priority inheritance could be enabled for pthread mutexes on macOS X, too. But before #310 RTT used the Boost.Thread mutex implementation to implement timed mutexes. With #310 Boost.Thread usage has been dropped, but because waiting on a mutex held by another thread is actually implemented by waiting on a condition variable, priority inheritance cannot easily be applied.