Improve install process/documentation

[gerrymanoim]

Relevant issues:

• Will not install on Ubuntu 16.04.6 LTS or 20.10 and Python 3.x #191
• Will not install under macOS (Python 3.8) #192

We should

1. Add a note to the docs that make will use python unless otherwise specified via an environment variable.
2. Fail to compile with a clearer error if PY_MAJOR_VERSION==2.