configuring out old ABI

[Rasmus Villemoes]

I naively attempted

  --disable-libstdcxx-dual-abi --with-default-libstdcxx-abi=cxx11

and was a little surprised to find out that the second option is
ignored, and I got the cxx98 ABI (only). Of course, it's documented if
one reads the fine print in libstdc++-v3/acinclude.m4

dnl --disable-libstdcxx-dual-abi will use old ABI for all types.

or the logic a little further down that only honours
default-libstdcxx-abi when x$enable_libstdcxx_dual_abi = xyes.

Is there any chance of having a configuration combination completely
disabling the old ABI?

Rasmus