Use actual recursive pthreads mutexes, rather than NIH'ing them, wrong

Use actual recursive pthreads mutexes, rather than NIH'ing them, wrong

Very loosely based on a patch from Sigmund Augdal.

For the moment, we make PTHREAD_MUTEX_RECURSIVE a hard requirement:
it's required by POSIX 2008 Base and SUSv2.

If your (non-Windows) platform doesn't have PTHREAD_MUTEX_RECURSIVE,
please report a bug on freedesktop.org bugzilla with details of the
platform in question.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=43744
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Thiago Macieira <thiago@kde.org>