maint: update to latest gnulib, for mingw improvements

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: Eric Blake <eblake@redhat.com>