Add a check to configure that the libstdc++ selected by Clang isn't
libstdc++v4.6. This is quite hard to test directly, so we test for it by
checking a known missing feature in that version that was added in v4.7.

This should prevent users from upgrading Clang but not GCC and hosting
with a too-old GCC's libstdc++ and getting strange and hard to debug
errors when we switch to C++11 by default.

Also, switch several of the macros I introduced to use AC_LANG_SOURCE
rather than AC_LANG_PROGRAM as we don't need configure's help writing
our main function (and we don't need such a function at all for most of
the tests).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199313 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a bug in r199313 where I failed to restore CXXFLAGS. Doh! Not
*quite* ready to just slam C++11 on by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199314 91177308-0d34-0410-b5e6-96231b3b80d8