build: fix virBufferVasprintf on mingw

Gnulib documents that mingw vsnprintf is broken (it returns -1
on out-of-space, instead of the count of what would have been
printed); but while we were using the snprintf wrapper, we had
not yet been using the vsnprintf wrapper.

Meanwhile, mingw (but not mingw64) has a replacement snprintf
that fixes return values, but still lacks %1$s support; so in
that case, gnulib didn't replace snprintf, but libintl then
went ahead and installed a version that supported %1$s but not
return values.  Gnulib has since been fixed to guarantee that
the snprintf module will always guarantee the constraints needed
by libintl.

Also, we want to guarantee that strdup sets errno on failure.

* .gnulib: Update to latest, for vsnprintf fix.
* bootstrap.conf (gnulib_modules): Add vsnprintf, strdup-posix.
Reported by Matthias Bolte.

bootstrap.conf

@@ -80,6 +80,7 @@ socket
 stdarg
 stpcpy
 strchrnul
+strdup-posix
 strndup
 strerror
 strerror_r-posix
@@ -97,6 +98,7 @@ usleep
 vasprintf
 verify
 vc-list-files
+vsnprintf
 waitpid
 warnings
 '