Add Github action for x86_64 Linux build

[ew1abz]

It will trigger a build every time you push code. It should be easy to add support for macOS and Windows later.

[wb2osz]

Interesting idea.
We should also add the "-DUNITTEST=1" option to cmake and "make test" at the end.

[dforsi]

This pull request can be closed because a Github actions were added in commit 0f92f46