Continuous integration via GitHub Actions.

[jdumas]

• Adds a PR check + nightly job via GitHub Actions.
• Runs on Windows/macOS/Linux. Currently missing: Android, Emscripten, MinGW and Linux-ICC
• Currently no unit tests are run. Not sure what you want to do there?

To facilitate CMake maintenance, I'd recommend the following:

• Get rid of configure.sh and configure.bat. Platforms can be detected directly via CMake (see new file geo_detect_platform.cmake.
• There are a lot of files in cmake/platforms.cmake, which seems like a pain to maintain. I'd recommend keeping a single platforms/Windows.cmake and a single platforms/Unix.cmake and use check_cxx_compiler_flag to check for compiler support for specific flags across all possible GCC/clang versions.

Continuous PR check:

Nightly check:

[BrunoLevy]

Fantastic Jeremie ! To make it clear, if I merge this pull request, it will start continuous integration directly ? There is nothing else to configure ?

[jdumas]

Yes I believe so. In the repo setting page you have some options related to github actions. By default it will require you to "approve" first-time contributors before running any action coming from a fork pull request.