Temporarily disable format truncation warnings

GCC 7.1 introduces a new -Wformat-truncation warning
flag that reports if it thinks the maximum possible
size of the formatted output will exceed the provided
fixed buffer. This is enabled automatically by the
-Wformat warning flag. There are quite a few places
hit by this in libvirt which need rewriting. This is
non-trivial work in some places, so temporarily
disable the new warning until those fixes can be
implemented.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
(cherry picked from commit f1acc413)

m4/virt-compile-warnings.m4

@@ -169,6 +169,10 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
       wantwarn="$wantwarn -Wno-format"
     fi
 
+    # -Wformat enables this by default, and we should keep it,
+    # but need to rewrite various areas of code first
+    wantwarn="$wantwarn -Wno-format-truncation"
+
     # This should be < 256 really. Currently we're down to 4096,
     # but using 1024 bytes sized buffers (mostly for virStrerror)
     # stops us from going down further