[C++11] Replace autoconf --enable-cxx11 with --enable-cxx1y. The
baseline is now C++11, and we unconditionally add -std=c++11 to the
flags.

This has the dim potential to break some non-GNU-compatible compiler (in
terms of -std flags) using the makefiles, but those makefiles are
littered with GNU-style compile flags so it would be very surprising to
me for it to actually happen in practice. As always, do let me know if
there is a toolchain you're using where this doesn't work, and I'll be
watching the bots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202569 91177308-0d34-0410-b5e6-96231b3b80d8