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.
In bd7d01e9, Meta/serenity.sh started checking cc and cxx if CC or CXX
respectively are not set in the environment. My machine does not have
cxx symlink in PATH, so every invocation of the script resulted in
the "cxx: command not found" message outputted to stderr, and that
became quite annoying.
