[MacPorts] #53634: clang++-mp-4.0 -stdlib=macports-libstdc++ should be in C++11 mode by default

[MacPorts]

#53634: clang++-mp-4.0 -stdlib=macports-libstdc++ should be in C++11 mode by
default
------------------------+----------------------
  Reporter:  mojca      |      Owner:  mojca
      Type:  defect     |     Status:  assigned
  Priority:  Low        |  Milestone:
 Component:  ports      |    Version:
Resolution:             |   Keywords:
      Port:  clang-4.0  |
------------------------+----------------------

Comment (by kencu):

 More I think about how this works, the more it seems a better idea to add
 {{{
 configure.cxxflags-append -std=c++11
 }}}

 in the right place in the cxx11 1.1 portgroup file rather than try to
 outsmart clang.

 Should also cover the option where the person has actually set
 `configure.cxx_stdlib macports-libstdc++` in macports.conf.

 and also not whitelist clang-4.0 on PPC, so solve a couple of tickets
 recently.

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Ticket URL: <https://trac.macports.org/ticket/53634#comment:7>
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