Add basic CI configuration for Linux, macOS and Android

[EwoutH]

Adds a basic CI configuration using GitHub Actions which builds sjpeg:

• on Linux using CMake and GCC 10
• on macOS using CMake and AppleClang 12
• for Android using CMake and the Android NDK

This configuration uses GitHub Actions, which needs to be activated here. An example build from my fork can be seen here.

For Windows there is already an AppVeyor configuration, which was added in #110.

[EwoutH]

@jzern @madebr @skal65535 I'm doing some cleanup of old, open PRs. If you would like to get these changes in, please leave a reply or review.

In two weeks I will close this PR if no reaction received.

[jzern]

Thanks for pinging this, I guess it got missed.