The logic in this script was *intended* to use the system's default
compiler if it was sufficiently new, and only start searching for the
latest installed if the default was not suitable.
However, the `cxx` program does not exist on Unixes, so the version
check always failed. We should be using the standard `c++` program name
instead.
After this change, the `CC` and `CXX` environment variables will have to
be used if someone wants to force a newer compiler version.