threads: update for late C11 changes

threads: update for late C11 changes

C11 threads were changed to use struct timespec instead of xtime, and
thrd_sleep got a second argument.

See http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1554.htm and
http://en.cppreference.com/w/c/thread/{thrd_sleep,cnd_timedwait,mtx_timedlock}

Note that cnd_timedwait is spec'd to be relative to TIME_UTC / CLOCK_REALTIME.

v2: Fix Windows build errors. Tested with a default Appveyor config
    that uses Visual Studio 2013. Judging from Brian's email and
    random internet sources, Visual Studio 2015 does have timespec
    and timespec_get, hence the _MSC_VER-based guard which I have
    not tested.

Cc: Jose Fonseca <jfonseca@vmware.com>
Cc: Brian Paul <brianp@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)