Problem:
- The version of clang to use is specified only as `clang++`. This is
inconsistent with the specifications for gcc and exposed to unexpected
failure if the default changes.
Solution:
- Specify the exact version of clang to use. I chose `clang++-9` as that
is the version that `clang++` is currently resolving to.
Co-authored-by: Jonathan Gopel <jgopel@quantlab.com>