This only applies to linux systems.  Here, sched_setscheduler() is
called to get realtime scheduling.  With this patch, the return value
of this function is now checked and a warning / error message is
generated if it fails.

Apparently all other C libraries are not compatible with "constexpr".
Those which are not will get a performance penalty, but at least they
work at all.

Caused a build failure with uClibc because snprintf() was not
available.

NetBSD's pthread_setname_np() prototype is incompatible with the rest
of the world, and it requires to pass the string argument as a
non-const pointer.  Instead of working around this misdesign, I hereby
disable the feature on NetBSD.

Add macro HAVE_THREAD_NAME which is set when any method to set the
thread name is available.  Use that macro in FormatThreadName()
instead of just checking for HAVE_PTHREAD_SETNAME_NP.

The "::" to explicitly refer to the global namespace appeared like a
good idea in C++, but it breaks with C libraries that implement
standard functions using macros (e.g. musl).

On NetBSD, PTHREAD_MUTEX_INITIALIZER and PTHREAD_COND_INITIALIZER are
not compatible with C++11 "constexpr" (see Mantis ticket 0004110).  As
a workaround, don't ues "constexpr", and use the functions
pthread_mutex_init(), pthread_mutex_destroy(), pthread_cond_init() and
pthread_cond_destroy() instead.  This adds some runtime overhead, but
is portable to POSIX implementations that have awkward initializer
macros.

The class does not have any virtual methods, and thus the (debug-only)
destructor does not need to be virtual.

Bionic doesn't have the SYS_* macros.

Use it if it exists, but don't insist if the C library is poor
(e.g. Bionic/Android).

Bionic's pthread declarations are non-literal.

Allows the kernel to combine timer wakeups with other processes,
reducing power usage.

pthread_setname_np() doesn't have the "thread" argument on OS X.

For debugging.

The update thread should not affect the rest of the system, therefore
set "idle" priority, and let it only run when nobody else is using the
resources.

Allows forward-declaration.

Remove a GLib dependencies from class EventLoop and DatabaseLock.

Use CRITICAL_SECTION and CONDITION_VARIABLE.  This requires Windows
Vista or newer.  It fixes problems with GLib threading objects that
were implicitly created by static constructors before g_thread_init().

Switch WIN32 to this implementation to be able to use condition
variables, which is impossible with CriticalSection.

Use a custom pthread_mutex_t wrapper because std::mutex adds overhead.