DBusAtomic: on Unix, use pthreads mutexes for fallback

DBusAtomic: on Unix, use pthreads mutexes for fallback

On pthreads platforms, POSIX guarantees that we can "allocate" mutexes
as library-global variables, without involving malloc. This means we
don't need to error-check their allocation - if the dynamic linker
succeeds, then we have enough memory for all our globals - which is an
important step towards being thread-safe by default. In particular,
making atomic operations never rely on DBusMutex means that we are free
to implement parts of DBusMutex in terms of DBusAtomic, if it would help.

We do not currently support any non-Windows platform that does not have
pthreads. This is unlikely to change.

On Windows, we already used real atomic operations; we can just
delete the unused global variable.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=54972
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>