Add CI builder

[jpcima]

This adds a builder with github actions.
There are builds on 2 distributions: the first one passes, and the second illustrates the linker problems which are corrected by PR #22.

[nomadbyte]

I understand that you got the link errors on Arch linux. I wonder what's different there vs Ubuntu/Debian in the way the linker resolving references?

[jpcima]

Having just checked the package comments, it would appear that these problems relate to gcc 10.

[nomadbyte]

I just cleaned up (in 3015be7) the use of brightonEvent event variable that was flagged in the linking on Arch. In both instances it was rather benign, however it was indeed incorrect use, as the event normally should be a local scope variable. Thanks for catching this.

Did you get similar link errors on the note_diff use too?

[jpcima]

Did you get similar link errors on the note_diff use too?

Yes, that one and lots of others, but all this is already fixed in the PR #22
I've made variables static wherever applicable.

[nomadbyte]

Turns out -fno-common is now a default in GCC 10.

[jpcima]

so if I get this right, these global variable must act as if they're shared?
Ok I should take a second look #22 to see if I preserve the behavior correctly.

On another note, is this CI stuff ok for merging?

[nomadbyte]

I have not decided yet on the way for packaging (or specific target platforms, if any); this will need some tweaking. So for now I postpone the automation of the builds and releases; after all, there's really no test suite yet to run to validate the pushes.