On platforms that don't support makecontext(3) use gthread based
coroutine implementation.

Darwin has makecontext(3) but getcontext(3) is stubbed out to return
ENOTSUP.  Andreas Färber <andreas.faerber@web.de> debugged this and
contributed the ./configure test which solves the issue for Darwin/ppc64
(and ppc) v10.5.

[Original patch by Aneesh, made consistent with coroutine-ucontext.c and
switched to GStaticPrivate by Stefan.  Tested on Linux and OpenBSD.]
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>