configure: Enable -Werror for MinGW builds, too

MinGW seems to compile currently without warnings, so it should
be safe to enable -Werror now for this environment, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1465373606-18486-1-git-send-email-thuth@redhat.com>
Tested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1390,7 +1390,7 @@ fi
 # by default.  Only enable by default for git builds
 if test -z "$werror" ; then
     if test -d "$source_path/.git" -a \
-        "$linux" = "yes" ; then
+        \( "$linux" = "yes" -o "$mingw32" = "yes" \) ; then
         werror="yes"
     else
         werror="no"