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).