build: use cmake to pass -std=c++11

Rather than manually checking for support for the spelling of the C++
standard, indicate to CMake that we require that the compiler support
C++11 and that we compile without the GNU extensions.  This simplifies
the flags handling in libc++abi itself by relying on CMake to translate
the flag and add it as appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@305175 91177308-0d34-0410-b5e6-96231b3b80d8