[PATCH] CMake/Autoconf : perform a sanity check on the compiler support for C++11

[Tim Northover]

> The logical follow-up to this patch would be to make the sanity check
> unconditionnal once we have decided to switch to C++11. Until then, the
> checks can be validated in the different environments and tuned accordingly.

Shouldn't we do that now, at least with a warning rather than an error?

Cheers.

Tim.