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Co-Authored-By: robbuckley <20515024+robbuckley@users.noreply.github.com>

Suggested by native-api...

[skip ci]

Path of env_vars.sh file should be relative to the repository, but when
we run in docker, the PWD is /, not /io, which is the repository root.

Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116

* Add config.pre to override envvars used in `library_builders.sh`
Fixes part 2 of https://github.com/matthew-brett/multibuild/issues/116

* rename config_pre.sh to env_vars.sh as suggested

Add notes clarifying what to check out
Addresses gh-38.

#111 dropped support

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116

* Add config.pre to override envvars used in `library_builders.sh`
Fixes part 2 of https://github.com/matthew-brett/multibuild/issues/116

* rename config_pre.sh to env_vars.sh as suggested

Add notes clarifying what to check out
Addresses gh-38.

#111 dropped support

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

Thanks to Moritz E. Beber for prompting me to formalize this.

The syntax is similar to how TravisCI does it.

Travis test uses `get_macpython_enviroment` for CPython and
`get_python_enviroment` for PyPy.

This changes how the the Python implementation is selected.  It's now
primarily based on enviroment variables.

Uses the PYPY_VERSION enviroment variable to determine the version of PyPy to
install.  An example has been added to the matrix in the readme.

Fixes #14.