[C++11] Switch autoconf and make to use C++11 by default. Now both build
systems have the default as C++11, but retain the ability to build with
C++98.

Again, please restrain your enthusiasm a bit in case this needs to be
reverted. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202546 91177308-0d34-0410-b5e6-96231b3b80d8