builds.
c2f69ae9fb added the use of the
`travis_wait` command to all builds but this stops interactive
build output from showing in the TravisCI web interface. To limit
the scope of this change we only use it for LTO builds now.
TravisCI kills builds that don't show output for over 10 minutes [1].
This sometimes causes LTO builds to fail because gcc shows no output
during linking which can take many minutes to complete.
To workaround this we use the `travis_wait` command to allow at
most 45 minutes for the build to run. This command will force output
to appear at regular intervals.
The change is made in the top-level `.travis.yml` file rather than
the other scripts because I don't want to pollute them with TravisCI
specific details.
[1] https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The Linux builds rely on Docker (using Ubuntu 16.04LTS and Ubuntu
14.04LTS) to build and test Z3 so that builds are easily reproducible.
A build status button has been added to `README.md` so that it is
easy to see the current build status.
More documentation can be found in `contrib/ci/README.md`.
This implementation currently tests 13 different configurations. If
build times become too long we can remove some of them.
Although it would be nice to test macOS builds that requires
significantly more work so I have left this as future work.
