The latest mingw headers on Fedora 19 fail to build with gnulib
without an update.
Meanwhile, now that upstream gnulib has better handling of -W
probing for clang, we can drop some of our own solutions in
favor of upstream; thus this reverts commit c1634100, "Correctly
detect warning flags with clang".
* .gnulib: Update to latest, for mingw and clang.
Signed-off-by: Eric Blake <eblake@redhat.com>