Xcode, and other, tweaks.

The main change for this snap release is dropping active support for use of the xcode-install tool to install Xcode releases from Apple Developer services. The security changes by Apple, i.e. 2FA, makes automated services access almost impossible. The alternative is that now there's added support for finding the desired, preinstalled, Xcode version directly by only using XCODE_VERSION environment variable.

Other tweaks include:

• Switched CircleCI to use Docker images instead of direct machine images for Ubuntu Bionic.
• Enable Xcode testing in Cirrus CI.
• Switch testing in Cirrus CI to use stable FreeBSD 12.

Now 8 CI providers.

This release now adds a somewhat working set up for Cirrus CI. It also means we now have a FreeBSD to test with. As of this release the following providers are tested: AppVeyor, Azure Pipelines, CircleCI, Cirrus CI, Drone, GitHub Actions, Semaphore, and Travis CI.

Initial release for 7 CI providers.

This is the beginning of CI Playground. With this initial release 7 cloud CI providers are tested for building C++. As of this release the following providers are tested: AppVeyor, Azure Pipelines, CircleCI, Drone, GitHub Actions, Semaphore, and Travis CI.