[Bug libstdc++/65762] New: --with-default-libstdcxx-abi=c++11 is silently ignored when --disable-libstdcxx-dual-abi is used

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65762

            Bug ID: 65762
           Summary: --with-default-libstdcxx-abi=c++11 is silently ignored
                    when --disable-libstdcxx-dual-abi is used
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

Ideally it should be possible to use only the new ABI (and that should be the
default for --enable-symvers=gnu-versioned-namespace) but it's not currently
supported.

Until it is supported, trying to configure the library that way should fail
with a clear explanation instead of appearing to configure OK and ignoring the
option.