maint: update to latest gnulib, for mingw improvements

On Fedora 20, mingw-headers has switched over to winpthreads as
the provider for its <pthread.h>.  winpthreads is notorious for
providing a less-than-stellar header, and needs several workarounds
before it can be used in a project assuming POSIX semantics.  While
we still use Windows primitives rather than pthread when compiling
for mingw, this update will make it possible to switch to mingw
pthreads.

* .gnulib: Update to latest, for mingw fixes.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ d5fec6c2

-Subproject commit d18d1b8023822220bb8f0a079c7312a1adffdce0
+Subproject commit d5fec6c22f03c6a73d62260c9ce091c10c0a9cbd