Have default builds use libc++ when clang is the selected compiler.

[greg7mdp]

Our pinned/reproducible builds use libc++.

In order for this library to be regularly used (and tested) by the Leap core developers, I feel we should make it the default option in our CMakeLists.txt when clang is the C++ compiler used.

[bhazzard]

Discussion needed on whether to use libc++ regardless of selected compiler, or only clang; also the implications to llvm.

The benefit is that we would be using what we are suggesting to users.