Command line arguments did not work.

Since LLVM/Clang 9, it is possible to link against dynamic libraries
instead of linking statically against LLVM and Clang.

The command line library works by accumulating static variables. This
fails on macOS when both the dynamic _and_ the static libraries are
linked together.

Make linking against the dynamic clang-cpp library optional.
Make sure that clang-cpp is not linked when linking the static clang
libraries.
Turn off usage of clang-cpp on macOS by default.

Amends 0e295e5a

Change-Id: I25ada3db8fd870d71b687cfaf2ab5d76a1206a76