pthreads-w32: Add patch for newer mingw-w64 versions

* Makefile.am: Add patch to EXTRA_DIST
* patches/pthreads-w32-2-8-0-release: Add
02-mingw_w64_timespec_compatibility.patch

The mingw-w64 header included in debian sid define timespec in time.h
for windows. As pthread has not a configurable build system we check
for a definition made by the header instead of checking for the
struct.

(cherry picked from commit 3e813b391bf77b91b4492aafc5c8f136a60d535f)