-Wextra as follows: "[class name] has a field [member name] whose type
uses the anonymous namespace", even if the anonymous namespqce in
question is closed in the single translation unit:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29365

AFAICS, the intent was to warn it if the anonymous namespace is used
in a header file (which can easily violate the One Definition Rule as
discussed in the bugzilla), but gcc seemingly did too much to prevent
that.

Unfortunately this bug prohibits us from specifying -Werror, so we
selectively disable this flag by identifying g++ used for build has
this bug.


git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1371 e5f2f494-b856-4b98-b285-d166d9295462