[Bug libstdc++/51705] New: Build infrastructure should not use -std=c++0x until g++ properly supports it

["ed at 80386 dot nl" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51705

             Bug #: 51705
           Summary: Build infrastructure should not use -std=c++0x until
                    g++ properly supports it
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ed@80386.nl


FreeBSD recently added support for C11 and C++11 keywords to its sys/cdefs.h
file. This header file is used by FreeBSD header files and source code to write
code that is portable across different compiler versions.

GCC 4.7's build system adds -std=c++0x to the compiler to build libstdc++. This
means that code is built with __cplusplus == 201103L. This is wrong, because
now sys/cdefs.h starts to use C++11 constructs such as [[noreturn]].
[[noreturn]] is not supported by GCC yet.

Essentially GCC is free to announce __cplusplus == 201103L for its experimental
C++11 support, but the C++11 support should not be used during its own
compilation step if it's not finished yet.

Alternatively, this could be fixed by simply implementing [[noreturn]], of
course, but that's a different issue altogether.