build: fix mingw ssize_t, syntax check

build: fix mingw ssize_t, syntax check

We are so close to a release that we don't want to pull in a
gnulib submodule update and risk regressions, since there has
been a lot of other gnulib churn upstream.  However, there are
a couple of gnulib issues that are worth fixing in isolation,
by applying local patches to gnulib.

There was an upstream gnulib bug in maint.mk that rendered most
of our syntax checks ineffective (and fixing it flushed out a
minor bug in our code):
https://lists.gnu.org/archive/html/bug-gnulib/2012-03/msg00194.html

There is still an upstream bug where gnulib uses the wrong type
for ssize_t on mingw; we need the fix now even though it has not
yet been accepted into gnulib:
https://lists.gnu.org/archive/html/bug-gnulib/2012-03/msg00188.html

* gnulib/local/top/maint.mk.diff: Pick up upstream gnulib
maint.mk.
* gnulib/local/m4/ssize_t.m4.diff: Work around gnulib bug.
* src/libvirt.c: Remove unused header.
* cfg.mk
(exclude_file_name_regexp--sc_prohibit_empty_lines_at_EOF): Exempt
gnulib local files.