CMake : optionaly enable LLVM to be compiled with -std=c++11 (default: off)

In some case, it may be required to build LLVM in C++11 mode, as some the subprojects (like lldb) requires it.

This mimics the autoconf behaviour.

However, given the discussions on the switch to C++11 of the codebase, this behaviour should evolve to default to C++11 with some checks of the compiler capabilities.