GitLab

* 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

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

Here, we pass both the USE_CCACHE variable and the CCACHE volume to docker.

Allow other build commands to Docker

BUILD_COMMIT not available directly from the repository, and may be useful for
later build steps.

Build / test Python versions conceptual separable in case of Linux
builds.

Make configuration more general.

Move responsibility for source futzing back to the travis.yml file,
because we are now doing only one build per job.