optional c++11 VS c++11 requirement of libxml++

[tschiemer]

As (at least newer versions of) libxml++ follow the c++11 standard builds where this is not c++11 compliance is not enabled by default will fail.

It might be worth a consideration to enable --force-cpp11 by default to avoid this error case.

[cth103]

Perhaps, yes. I guess the default should be right for the majority. I'm not sure whether the majority are using a new enough libxml++ though...

[tschiemer]

Well, I would go for the way where builds break with the least probability.

That is, if it doesn't hurt (break) to have c++11 enabled by default, then I would enable it and add the option to disable it instead.
->ie the question: does your typical compiler support c++11 or are there other components breaking with it?

Otherwise I'd leave it as is, but add a comment in the readme (or a version check in the build system)