Add lldb to the build

This optionally adds lldb (and clang, which it needs) to the build.

Because rust uses LLVM 7, and because clang 7 is not yet released, a
recent git master version of clang is used.

The lldb that is used includes the Rust plugin.

lldb is only built when asked for, or when doing a nightly build on
macOS.  Only macOS is done for now due to difficulties with the Python
dependency.