GitLab

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

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container

* [osx] don't update homebrew

* [docker] move activate_ccache until after utils are loaded

* [manylinux_utils] create cccache directory

* [manylinux_utils] fix case

* [osx/ccache] disable maxsize

* [osx_utils] move  code and add comment

Link the docker ccache directory to the VM; install and verify that ccache is operating correctly

Allow other build commands to Docker

library_builders.sh script in fact needs common_utils.sh.  Source that
script, and put a guard in common_utils.sh to prevent it being sourced
more than once.

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

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.